A Defense of Naturalism
University of Maryland, College Park in partial fulfillment
of the requirements for the degree of
Master of Arts
Professor Raymond F. Martin, Chair
Professor Allen Stairs
Professor Corey Washington
Extraordinary Claims Require Extraordinary Evidence
Metaphysical and Methodological Naturalism
A Natural History of the Universe
Naturalism and a Scientific Picture of the World
The Status of Parapsychological Research
Throughout the twentieth century naturalism has been a label for a variety of distinct positions which have little, if anything, in common. In ethics, naturalism is a form of moral realism which contends that ethical properties are objective in virtue of being reducible to or identical to natural properties, where natural properties are simply the properties investigated by various sciences. In metaphysics, naturalism typically takes a form of materialism or physicalism: Everything that exists is either physical or supervenient upon the physical. Naturalism in epistemology contends that the role of epistemology is to describe how knowledge is obtained rather than to set out a priori criteria for the justification of beliefs; thereby a naturalized epistemology provides theories of knowledge and justification which eliminate normative standards by using only scientific concepts.
In this essay I will be concerned with naturalism in the philosophy of religion, where other basic metaphysical and epistemological issues will arise. Naturalism in this domain is the antithesis of supernaturalism--it is often construed as the view that everything that exists is natural and thus by implication that the supernatural does not exist. Behind this superficially simple formulation of naturalism, however, lies a wealth of implicit complexity. Part of this complexity consists of the analysis of the meaning of the word 'nature' or 'natural', how nature is to be characterized, and how the natural-supernatural distinction is to be drawn, both in theory and in practice. These issues will be addressed in the first part of this essay. In the second part I will defend naturalism as a more reasonable option for belief than either supernaturalism or agnosticism. This defense of naturalism will rest on an argument which contends that the lack of uncontroversial evidence for potential instances of supernatural causation provides strong inductive grounds for taking naturalism to be true.
One of the most common versions of naturalism is the position that everything that exists is natural. Robert Audi defines naturalism, broadly construed, as "the view that nature is all there is and all basic truths are truths of nature" (Audi 1996, p. 372). Rem B. Edwards offers a similar definition: "[T]he naturalist is one who affirms that only nature exists and by implication that the supernatural does not exist... The [natural] world is all of reality; it is all there is; there is no 'other world' " (Edwards 1972, p. 135). Although these definitions capture some of the most fundamental features of naturalism, I think that naturalism can be--and thus should be--defined less strongly. Alan Lacey captures the heart of naturalism when he writes: "What [naturalism] insists on is that the world of nature should form a single sphere without incursions from outside by souls or spirits, divine or human" (Lacey 1995, p. 604).
I think that most naturalists would agree that naturalism at least entails that nature is a closed system containing only natural causes and their effects. Fundamentally, naturalism is a metaphysical position about what sorts of causal relations exist--it is the position that every caused event within the natural world has a natural cause. This definition of naturalism is weaker than "everything that exists is natural" because it leaves open the possibility that the natural world does not exhaust all of reality: There may be some aspects of reality which exist outside of nature. Which aspects of reality are nonnatural in this sense will vary with the different definitions of nature or natural being used. It may even be impossible in principle to know that such nonnatural realms exist. But this weaker definition retains the fundamental core of naturalism by denying that supernatural causation exists. It would thus be better to say that naturalism is the position that everything that exists within nature is itself natural and is solely influenced by natural causes.
Naturalism, as I conceive it, thus allows the existence of both nature and realms that may exist outside of nature; it simply stipulates that any nonnatural realms which may exist cannot causally influence the natural world. Even the possibility of nonnatural causation is not ruled out so long as both the cause and effect reside in some nonnatural realm. Thus naturalism allows for the existence of both the natural and the nonnatural--including instances of natural and nonnatural causation--as long as these domains are causally separate. A supernatural cause, on this view, would be a nonnatural cause of an event within nature. The phrase 'supernatural event' is best taken to refer to an event within nature which has a supernatural cause. The phrase 'natural event' can refer to either an event with a natural cause or an event in the natural world. We should distinguish between these two, so I will not use the phrase 'natural event'. Instead, I will use the phrases 'naturally-caused event' and 'event within nature' (or the natural world), respectively, to mark this distinction. Naturalism is thus best construed as the denial of the existence of any genuine instances of supernatural causation, whereas supernaturalism is the affirmation of the existence of such instances.
Arthur C. Danto comes closest to explicitly defining naturalism in this way when he characterizes naturalism as entailing that "The entire knowable universe is composed of natural objects--that is, objects which come into and pass out of existence in consequence of the operation of 'natural causes' " (Danto 1972, p. 448). But what is a natural cause? According to Danto,
A natural cause is a natural object or an episode in the history of a natural object which brings about a change in some other natural object... [I]t is solely with reference to natural causes that we explain changes in the behavior of natural objects. This may require reference to objects which we cannot directly experience, but these will nevertheless still be natural objects, and we need never go outside the system of natural objects for explanations of what takes place within it. Reference to nonnatural objects is never explanatory (Danto 1972, p. 448).
Insofar as the meaning of the term 'natural' is not made explicit, the definition above leaves open the possibility that 'natural cause' might be defined broadly as any cause of a change in the behavior of a natural object. Such a broad definition of 'natural cause' clearly begs the question: That all causes of events within nature are natural causes is precisely the issue in question. We certainly don't want this thesis to be true by definition--that is, true in a trivial sense. Rather, we want naturalism to be a position which--if true--is informative. The poignant feature of Danto's definition which seems most essential to naturalism is the thesis that we never need to look to something outside of the natural world to explain anything within the natural world.
On Danto's definition, we may not always be able to directly experience a natural cause, but presumably we should be able to experience it indirectly, as when we think of atoms as natural objects. While Danto never states how he distinguishes between directly experiencing an object and indirectly experiencing it, I will presume that he means something like the following: An object is directly experienced if it is immediately present to our senses; it is indirectly experienced if we must infer its presence to explain the behavior of other objects which are immediately present to our senses. Danto's discussion of nonnatural objects indicates that he does not intend 'natural cause' to refer simply to any cause of a change in a natural object:
The universe may in addition contain one or another sort of nonnatural object, but we have no reason for allowing the existence of nonnatural objects unless they have impact on the observable behavior of natural objects, for natural objects are the only objects about which we know directly, and it would be only with reference to their perturbations that we might secure indirect knowledge of nonnatural objects, should there be any (Danto 1972, p. 448).
Suppose we grant Danto his assumption that only natural objects can be known directly. A crucial question still arises: Among indirectly-known objects, how do we distinguish between those which are natural and those which are nonnatural?
Danto's definition of a natural cause, while capturing very general features of natural causation and natural causal explanation, does not shed much light on what is meant by the term 'natural' itself. One obvious candidate for what is meant by the term 'natural' is physical. The earliest forms of naturalism, in fact, were versions of materialism or physicalism which maintained that everything that exists is physical. As I have construed naturalism, simple (reductive) physicalism maintains that everything that exists within nature is physical and solely influenced by physical causes. However, the prominent twentieth century debate over materialism in the philosophy of mind has revealed several difficulties with reductive physicalism as a solution to the mind-body problem.
One of the most persistent difficulties for reductive physicalism has been the apparent inability of physicalistic explanations to capture qualitative features of conscious experience. It has been persuasively argued that qualia--the experiential feels of 'what it is like' to be in a conscious mental state--cannot be captured by any physicalistic explanations in principle because physicalistic explanations inherently refer to objective or public features of phenomena, whereas the experiential features of consciousness are inherently subjective or private (Teller 1992, pp. 190-191). While such arguments for the irreducibility of consciousness are not the last word on the subject, they have not been decisively refuted either--at least not in the view of several prominent philosophers. Although such difficulties may be resolved in the future, their current resistance to a clear resolution that gains widespread acceptance gives us good reason to resist simply identifying the natural with the physical.
In the contemporary philosophy of mind, an attractive alternative to reductive physicalism is some version of nonreductive physicalism or property dualism. According to nonreductive physicalism, mental states are not simply identical to certain physical states (such as brain states), as reductive physicalists hold; rather, mental states are supervenient upon those physical states. There have been several competing definitions of supervenience suggested in the philosophical literature. In general, however, to say that mental states supervene upon physical states is to say that there can be no differences between mental states without a physical difference between the objects which instantiate those states (Beckermann 1992, p. 11). This physical difference usually amounts to a difference in brain states, though the same mental states may be supervenient upon the physical states of an advanced computer or of an extraterrestrial brain. For our purposes, it is sufficient to say that for a mental state to be supervenient upon a physical state entails that a mental state is dependent upon and determined by that physical state without necessarily being identical to it.
But if mental states are supervenient upon some physical states and are not identical to any physical states, this means that mental states are--by definition--nonphysical. If we accept nonreductive physicalism (or even admit it as a reasonable position) and want to retain naturalism, we do not want to say that 'natural' is simply equivalent to 'physical'. However, the driving idea behind nonreductive physicalism allows us to consider another candidate for the natural: perhaps the term 'natural' means physical or supervenient upon the physical. On my definition of naturalism, nonreductive physicalism maintains that everything that exists within nature is either physical or supervenient upon the physical and solely influenced by physical causes or causes which are supervenient upon physical causes. A more economical statement of this form of naturalism would drop the idea of supervenient causation: everything that exists within nature is either physical or supervenient upon the physical and solely influenced by physical causes. Most reductive and nonreductive physicalists alike subscribe to the causal closure of the physical--the view that all caused events in the physical world must have physical causes (Van Gulick 1992, p. 160). Moreover, nonphysical causation is unlikely given that the brain would behave noticeably differently under the constant influence of nonphysical causes than it would in the absence of such influence and we see no evidence for nonphysical influences on the brain.
If naturalism is construed as the position that everything that exists is natural, the definition of natural as 'physical or supervenient upon the physical'--though initially promising--runs into potential difficulties. Consider the philosophical debate over the existence of abstract objects. According to Platonism, there exists a class of mind-independent entities called abstract objects (Hale 1987, p. 11). On traditional Platonic accounts, abstract objects are immutable and timeless entities which are incapable of being involved in causal interactions--that is, are acausal--because they exist outside of space and time in a Platonic realm of unchanging and eternal forms. A paradigm candidate for a genuine abstract object is a number:
Numbers, sets and other stock examples of the abstract have neither spatial nor temporal position. Someone who seriously persisted in asking after the whereabouts of the number 3, say, or when it began to exist, or how long it will endure, etc., could only be supposed to be the victim of a gross misconception concerning what kind of thing numbers are (or are taken to be). With such paradigmatic examples of the abstract in mind, it is natural to propose that the distinguishing feature of abstract objects is lack of spatial or temporal location (Hale 1987, p. 48).
However, it is questionable whether Platonism must be characterized in this way. For example, Bob Hale points out that while all candidates for abstract objects are nonspatial, certain candidates for abstract objects, such as the game of chess and the English language, have an origin in time (Hale 1987, p. 49). One could argue that such examples are not genuine abstract objects after all, though Hale thinks that this is implausible. Despite this assessment, however, Hale does concede that "the vast majority of abstract objects surely are wholly atemporal as well as non-spatial" (Hale 1987, p. 253). Perhaps the only abstract objects which we are forced to countenance as real, if we are forced to countenance any at all, are those which clearly exist outside of space and time. This would explain why abstract objects are in some sense acausal. Hale points out that while it isn't obvious that abstract objects must be completely acausal, "when abstract objects are said to be constitutionally incapable of causal involvement, what is meant is that they cannot be causes of change, and perhaps also that they cannot undergo change" (Hale 1987, p. 2). Given Danto's understanding of a cause as something "which brings about a change" in an object, abstract objects are acausal in the sense of causality that we are interested in.
In any case, I will confine our exploration of the controversy over abstract objects to paradigm cases of abstract objects like numbers where the traditional definition of abstract objects does apply. There is nothing we can point to within space and time and say 'that is the number 4'. Furthermore, numbers and the relations between them are unchanging and mathematical truths like 2+2=4 seem timelessly true. Physical objects such as acorns can be arranged such that we can say that there are only four of those objects within a given space, but these objects exemplify instances of the number 4--they are not equivalent to '4' itself. On a Platonic account, four acorns are a concrete and particular exemplification of this abstract and universal form. So 4 is a universal concept rather than a particular one. The number 4 is also an abstract concept rather than a concrete one, unlike the idea of an acorn. We cannot point to the number 4 in the way we can point to an acorn--this is the essence of what being an abstract object is.
Does naturalism allow the existence of abstract objects? Alan Lacey thinks that naturalism construes the natural world as a closed system of natural causes and effects "without having to accommodate strange entities like non-natural values or substantive abstract universals" (Lacey 1995, p. 604). Similarly, Arthur C. Danto thinks that naturalism entails the denial of the existence of abstract objects. Danto argues that formal sciences like mathematics
no more entail a Platonistic ontology than [the empirical sciences do], nor are we, in using algorithms, committed to the existence of numerical entities as nonnatural objects. If the formal sciences are about anything, it will at least not be a realm of timeless numerical essences, and at any rate logic and mathematics are properly appreciated in terms not of subject matter but of function, as instruments for coping with this world rather than as descriptions of another one (Danto 1972, p. 449).
Robert Audi, by contrast, thinks that naturalists can admit the existence of abstract objects, noting that they would still be naturalists 'about the world': "[A] naturalist does not have to be a radical physicalist--taking the position that only physical phenomena are real, not even excepting such well-behaved abstract entities as sets" (Audi 2000, p. 31). Audi argues that abstract objects may be essential for any adequate ontology: "It is even more obvious that it could turn out to be impossible to give an adequate account of science, not to mention philosophy, without positing some kinds of abstract entities, such as numbers, propositions, and possible worlds" (Audi 2000, p. 32).
What grounds do we have to believe that abstract objects actually exist? There has been a persuasive argument that at least some abstract entities, particularly mathematical objects such as numbers and sets, are indispensable to our best scientific theories (Hale 1987, p. 104). Insofar as our best scientific theories rely on mathematical descriptions which (allegedly) presuppose the existence of mathematical abstract objects and we ought to believe those theories, we ought to (so the argument goes) accept the existence of at least some abstract objects. The grounds for belief in abstract objects, on this view, are equivalent to the grounds we have for believing in the existence of theoretical entities essential to physics, such as electrons. Of course there are also grounds for doubting the existence of any abstract objects. A common nominalist objection is that by their acausal nature abstract objects are impossible to detect (directly or indirectly) in principle and thus it would be impossible for us to have knowledge of them even if they did exist (Hale 1987, p. 79). Many nominalists conclude that since Platonists do claim to have knowledge of abstract objects and such knowledge is impossible in principle, the concept of an abstract object is incoherent and thus abstract objects do not exist.
At this point in history there is no clear resolution to the question of whether abstract objects exist. However, many philosophers believe that we must admit the existence of at least some Platonic abstract objects if we are going to provide an adequate account of the world. Because intuitively, at least, there do seem to be abstract objects which are acausal and exist outside of space and time, we have grounds for thinking that the 'physical or supervenient upon the physical' criterion for what it means to be natural may be deficient. Such a criterion will certainly not allow for the existence of natural abstract objects. While it is still an open question whether such objects actually exist, if we do want to admit the possibility of their existence on this criterion we would have to admit that such objects (if real) are nonnatural.
Because abstract objects are acausal, at least in the sense of not being causes of change, they cannot be supernatural. But depending on how strongly you characterize naturalism, abstract objects may or may not count as part of reality. If naturalism means that everything is natural and natural means "physical or supervenient upon the physical", then naturalism entails a denial of the existence of abstract objects since abstract objects are neither physical nor supervenient upon anything physical. However, if naturalism merely entails that everything that exists within nature is itself natural and is solely influenced by natural causes, then naturalism can admit the existence of nonnatural abstract objects which exist outside of nature and do not causally influence the natural world. Arguably, the issues involved in the Platonic realism-nominalism debate should be settled on grounds independent of the truth or falsity of naturalism or how naturalism is characterized.
Even if we accept a weaker definition of naturalism which allows for the existence of nonnatural abstract objects and we want to say that 'natural' is equivalent to 'physical or supervenient upon the physical', it isn't clear what 'physical' itself means. In order to determine what 'physical' means it seems that we have to identify characteristics that all physical things have in common--that is, we have to identify fundamental physical properties. Anything belonging to the category 'physical' would at least exhibit those properties. Identifying properties common to all physical objects does not seem too daunting a task: All physical objects seem to have mass, for example. But physical objects do not exhaust the category 'physical'; the category also includes forms of energy, types of events, physical processes, and even spacetime itself. William P. Alston points out how difficult it is to determine what it means for something to be physical:
I take it to be reasonably clear what it is for a substance to be a material substance. Being spatially extended (plus, perhaps, having certain fundamental physical properties like mass) would seem to be necessary and sufficient. But what it is to be a material (physical) state, property, process, or event presents considerably more difficulty. States and properties (and perhaps events and processes as well) are not susceptible to spatial extension in the clear, unproblematic way substances are... [W]e might say that a state, event, or process is physical if it is definable as the exemplification (by a physical substance?) of one or more physical properties (Alston 2000).
But if we label something physical because it exemplifies physical properties we are back to where we started--we still have not said what it means for a property to be physical. While we can list several clear-cut examples of physical properties (e.g. mass, electric charge, gravitational attraction), we cannot say what it is about those properties that makes them physical.
The possibility of property dualism as a solution to the mind-body problem also makes it impossible to identify physical properties simply as the properties of physical substances--arguably, some physical substances (such as functioning brains) will have nonphysical mental properties. Alston sees the debate between reductive physicalists and property dualists as evidence of a lack of any clear notion of the physical:
What is our concept of 'physical state' such that it is a sensible question as to whether my believing that p or my being visually presented with a maple tree is or is not a physical state, given that I do not recognize the possibility of a non-physical substance of which it is a state? Spatial location cannot be the issue. States of me are not spatially located in the way in which I am. Again, it may be said that the question is as to whether the property of believing that p is a physical property. But what is that issue? What does it take to make a property physical? Presumably being a property of a physical substance is not sufficient, or there would be no controversy here. So what are the pure materialist and the property dualist arguing about? (Alston 2000).
Alston also points out that physical forces have physical properties yet do not necessarily belong to an extended physical substance.
This leads us to an alternative possibility: Perhaps we should define 'physical' as that which constitutes the subject matter of the physical sciences. That is, that which is physical is that which physics countenances as part of its subject matter. There are at least two problems with this suggestion, however. First, certain concepts (such as mental categories) may be countenanced into physical science in the future, although they are not now:
If... we do not characterize the physical intrinsically in terms of some kind of property, we appear forced to define it by appeal to what physical scientists discover, or perhaps would ultimately discover. Then we cannot know a priori that, for example, irreducibly mentalistic explanations will not ultimately be part of what the people we call physicists consider their best overall account of reality (Audi 1996, p. 373).
Second, by defining 'physical' as what physicists study, we make an ontological category relative to the state of scientific knowledge:
When we define the physical by reference to what physical scientists think and work with, what period of the history of science are we to pick for this purpose? It seems arbitrary to canonize the present moment as defining a central metaphysical category. But if we make the definition relative to the stage of scientific development we are considering, then the metaphysical issues change with each fundamental shift in science (Alston 2000).
If abstract objects exist we can still maintain that the category 'natural' is equivalent to 'physical or supervenient upon the physical' by arguing that abstract objects are nonnatural. The possible existence of abstract objects only gives us reason to construe naturalism less strongly than it often is, admitting the possible existence of the nonnatural (but not the supernatural). However, as the discussion above illustrates, it is far from clear what 'physical' itself means. The best we may be able to do is say what sorts of things are clearly physical and what sorts of things appear to be nonphysical and admit that we have no clear criteria for distinguishing between the two. Perhaps this is an ambiguity we can live with--after all, as we soon shall see, there is going to be ambiguity in every definition of 'natural' considered. Nevertheless, this gives us some reason to consider other potential candidates for the category 'natural' in the hope that they will be less ambiguous than 'physical or supervenient upon the physical'.
Another candidate for what the category 'natural' refers to is the spatiotemporal--the 'natural' is that which exists within space and time. On this criterion, for an object to be natural it must have spatial extension and temporal duration; thus abstract objects are nonnatural on this criterion as well. Danto claims that "[e]very natural object exists within the spatiotemporal and the causal orders" (Danto 1972, p. 448). Similarly, Rem B. Edwards defines nature as "the spatiotemporal universe as a whole existing independently of [a] knowing mind" (Edwards 1972, p. 135). Edwards also implies that 'natural' is synonymous with 'spatiotemporal' when he claims that naturalism entails the following: "[A]ll natural events have causes that are themselves natural events. The occurrence of every spatiotemporal event is caused by some other spatiotemporal event or events" (Edwards 1972, p. 136). The fundamental dispute between naturalists and supernaturalists, according to Herbert Spiegelberg, is whether the spatiotemporal exhausts all of reality:
Both [naturalists and supernaturalists] seem to understand by 'nature' the sum total of all occurrents in time and space which are (1) explainable by other occurrents within the same order... and which are (2) usually, though perhaps not necessarily, subject to change... The issue between the two camps would thus seem to be whether nature so defined exhausts the content of reality or whether other entities are to be added for a complete account of the universe (Spiegelberg 1951, p. 342).
An immediate concern is that it is not clear how the spatiotemporal differs from the physical. Is the spatiotemporal simply equivalent to the physical? We do not seem to distinguish between the two terms in everyday usage. However, physicists have speculated about the possible existence of physical entities which are not spatiotemporal. For example, cosmologists have postulated the existence of a singularity--a point of infinite density where the known laws of physics break down--at the beginning of the universe and also believe that singularities lie at the center of black holes. According to Kip Thorne, somewhere near the singularity the unknown laws of quantum gravity take over, replacing spacetime with an exotic quantum foam:
Quantum gravity then radically changes the character of spacetime: It ruptures the unification of space and time into spacetime. It unglues space and time from each other, and then destroys time as a concept and destroys the definiteness of space. Time ceases to exist; no longer can we say that 'this thing happens before that one,' because without time, there is no concept of 'before' or 'after.' Space... becomes a random, probabilistic froth (Thorne 1994, pp. 476-77).
Quantum foam is a theoretical instance of something physical though not spatiotemporal. Furthermore, there may be other universes than the one we inhabit. If we presume that different universes are causally isolated from each other and space and time are merely specific instances of dimensions, it is possible that there will be other universes which are not spatiotemporal but have a different sort of dimensional existence which we cannot even begin to fathom. Moreover, if other universes are spatiotemporal, their spatiotemporal framework will not be our spatiotemporal framework--other universes may have different histories, different laws of physics, and different dimensions than our own. Universes may 'bubble off' from a quantum foam in a sort of 'metauniverse' from which all universes originate--but then that metauniverse will not itself be spatiotemporal.
I imagine that our notion of the natural and the physical should be broad enough to encompass singularities, other universes, a possible metauniverse, and other exotic possibilities raised in cosmology. 'Physical' seems to encompass more than the familiar dimensions of space and time and its contents--it will certainly cover any other dimensions that physicists may postulate. Thus the spatiotemporal seems to fall within the larger domain of the physical. If the physical is too restrictive a category to be identified with the natural, and the physical is broader in scope than the spatiotemporal, then the spatiotemporal criterion would also exclude things we want to count as natural. Thus 'natural' should not be identified with 'spatiotemporal'.
Perhaps by 'natural' we should mean something with a more epistemological flavor than the previous criteria we have considered. Danto also characterizes the natural in terms of scientific knowledge rather than solely in terms of ontological status:
Naturalism... is a species of philosophical monism according to which whatever exists or happens is natural in the sense of being susceptible to explanation through methods... paradigmatically exemplified in the natural sciences... Hence, naturalism is polemically defined as repudiating the view that there exists or could exist any entities or events which lie, in principle, beyond the scope of scientific investigation (Danto 1972, p. 448).
On this criterion, the natural is that which is amenable in principle to scientific investigation. Abstract objects, if they exist, are not scientifically accessible in the way that our paradigm case of natural objects--physical objects--are, and thus should fall within the domain of the nonnatural here as well. Note that on this criterion naturalism is still construed as a metaphysical position even though it is cast in epistemological terms: it is the position that everything that exists within nature is amenable in principle to scientific investigation and solely influenced by causes which are amenable in principle to scientific investigation. This invokes metaphysical claims about what exists within nature and what sorts of causes influence events within nature but characterizes its ontology in epistemological terms as what can be explained scientifically. Presumably, something is amenable in principle to scientific investigation in virtue of some more basic property or properties. However, this criterion alone does not spell out what those properties may be. In my final candidate for the 'natural'--the candidate immediately following this one--I will consider such a property. But for now I will construe the 'amenable in principle to scientific investigation' criterion without spelling out what properties something must have to be amenable in this sense.
There are many phenomena which are not amenable to scientific investigation given our current state of knowledge but intuitively we want to say that some phenomena are clearly scientifically explicable in principle even if we cannot explain them given our current state of knowledge. For example, it isn't clear what happens to a piece of matter when it falls into a black hole but we don't want to say that such a question is scientifically inexplicable in principle. It is simply inexplicable given our current state of physical theory. A future theory of quantum gravity may yield confirmed testable predictions which would allow us to answer that question by determining what the theory predicts about such situations.
However, there are other phenomena which do appear to be scientifically inexplicable in principle. Qualia, for example, resist current scientific explanations because they seem to be inherently private or subjective, whereas scientific explanations seem to require, as a matter of principle, explanations in terms of objective or publicly-accessible features of the world. While the qualia problem may ultimately be resolved, perhaps by denying that qualia really are inherently subjective or by denying that scientific explanations must be cast in terms of objective features of the world, at this point there is no obvious solution in sight. Since biological organisms which are clearly part of the natural world are the subjects of qualitative states we should categorize qualia as falling within the domain of the natural. If there can be no scientific explanation of qualia in principle--which is a tenable position--and we want countenance qualia as part of the natural world, we should not simply identify the natural with that which is amenable in principle to scientific investigation.
One might also object that if there are any other universes which are causally isolated from the universe we inhabit, by definition the contents of these universes would be inaccessible to scientific investigation. Yet we would probably want to countenance any other universes, should there be any, as part of the natural realm. This argument is not as persuasive as the qualia example, however, because the contents of causally-isolated universes would not be amenable in principle to scientific investigation only to us, not to any inhabitants of those particular universes. Nevertheless, the possibility of scientifically inexplicable qualia gives us some reason to resist identifying the natural with that which is amenable in principle to scientific investigation.
In addition to this theoretical problem, it is immediately apparent that this criterion is problematic in a fundamental way: by including the qualifier 'in principle' the criterion is too ambiguous, as it stands, to be of much use. Essentially it appears impossible to say a priori whether or not a given phenomenon is amenable in principle to scientific investigation. The best we can do is say that phenomena for which we have successful scientific explanations are definitely amenable in principle to scientific investigation since they are so amenable in practice. But for phenomena which resist scientific explanation, we cannot say categorically that they are or are not scientifically explicable in principle. The only way that we could determine that a phenomenon is scientifically explicable in principle is by providing a successful scientific explanation of that phenomenon. The problem is that this approach provides an a posteriori answer to such questions when we need an a priori criterion for determining whether something is amenable to scientific investigation in principle.
I do not see any way out of this dilemma. This gives us good reason to abandon 'amenable in principle to scientific investigation' as the sole criterion for the natural. However, if we put qualia aside for the moment, there still may be hope for the criterion if we can find some more basic property or properties by which something is amenable to scientific investigation in principle. By spelling out what properties make an object or event amenable in principle to scientific investigation we may be able to avoid this dilemma.
The final criterion for what makes something natural is that it behaves in accordance with laws of nature. Natural causes are clearly amenable to empirical investigation in principle because they exhibit exceptionless or probabilistic regularities which can be described by laws of nature. Laws of nature are factual statements which accurately describe the behavior of natural objects. By definition, natural laws never change and apply to everything within nature. It is the very existence of lawful regularities which allows us to form causal relationships. If laws of nature did not exist, we could not describe some events as causes and others as effects; the concept of natural causation would have no meaning because events would occur randomly without any recognizable pattern. Any phenomenon which is amenable to scientific investigation can be framed in terms of natural causal relationships established by the discovery of lawful regularities. Thus behaving in accordance with natural laws is a more fundamental criterion for the natural than being amenable in principle to scientific investigation: obeying laws of nature is a prerequisite for being amenable to scientific investigation. Naturalism, on this criterion, contends that everything that exists within nature behaves in accordance with laws of nature and is solely influenced by causes which obey natural laws.
On this criterion--like all the others we have considered--abstract objects are nonnatural. Anything which does not obey laws of nature is nonnatural. Abstract objects do not obey natural laws because they do not exist within nature--the laws of nature simply do not apply to them. However, the laws of nature do apply to all natural objects and events. An event within the natural world which has a cause that does not obey the laws of nature is an event with a supernatural cause. If we exclude the possibility of causal overdetermination for the sake of explanatory simplicity--that is, we exclude the possibility that a supernatural entity could cause an event which could be explained solely in terms of natural causes--then any event within nature which resulted from a supernatural cause would behave in a way which was not in accordance with the laws of nature.
If natural causes are sufficient to bring about an event then an appeal to supernatural causation to explain such an event is at least prima facie unwarranted since it is unnecessary and uninformative. If we defined supernatural causation in such a way that it fell within the scope of purely natural causation it would be impossible to provide a reliable basis for distinguishing between an event with a natural cause from an event with a supernatural cause. Thus, we should assume that a supernatural event--any event within nature (and thus within the domain of laws of nature) which had a supernatural cause--would be a violation of a law of nature. This retains the fundamental idea of the supernatural as an intervention into nature. A phenomenon which does not involve the violation of laws of nature by definition is a natural phenomenon. A supernatural event--such as the parting of the Red Sea as described in the Old Testament--is still presumably governed by some laws of nature: for example, the water still coheres together and holds most of its properties. However, some nonnatural force seems to counteract the natural forces in place, and this amounts to a violation of a law of nature.
More needs to be said about how natural laws themselves are to be understood. I will distinguish actual laws of nature from scientific laws on the following assumption: Scientific laws (such as laws of physics) are inaccurate but reasonable approximations of genuine laws of nature (Armstrong 1983, p. 6). I will also assume a necessitarian account of laws of nature--an account which contends that the actual laws of nature accurately describe physical necessities in nature, whether those necessities are properties of laws themselves or inhere in the constituents of the universe (Leckey and Bigelow 1995, p. 92). This contrasts with a regularity account of laws of nature, according to which laws of nature simply accurately describe which sorts of events happen in the entire history of the universe (Armstrong 1983, p. 32). The difference between these views is that necessitarian accounts imply that laws of nature 'forbid' certain events from happening--the laws of nature 'dictate' that they cannot happen. Events which cannot happen according to natural laws--that is, violations of laws of nature--are physically impossible events. Regularity accounts of laws of nature deny that there is any sort of physical possibility--there is only the logically possible and the purely contingent. A negation of a natural law is simply a description of a logically possible event that never happens in the entire history of the universe.
A necessitarian might argue that any spacecraft we develop cannot travel faster than the speed of light because that would be a physically impossible event--the laws of nature forbid such an event--it cannot happen. A regularity theorist would argue that such an event simply never occurs in the history of the universe--it simply does not happen. On a necessitarian account of laws of nature, such an event would be physically impossible, but on a regularity account, any logically possible event could happen--it just turns out that in the entire history of the universe certain events do not happen even though they could have happened. A major difficulty for regularity accounts of laws of nature is that they fail to distinguish between genuine laws of nature and merely accidental generalizations. For example, on a necessitarian account, it is a physically necessary truth "that all solid spheres of enriched uranium have a diameter of less than one mile" because it would be physically impossible, given the physical properties of enriched uranium, for such a sphere to exist (Weinert 1995, p. 18). This is neither a logically necessary truth nor an accidental truth. It is true in virtue of the laws of nature. By contrast, "that all solid spheres of gold have a diameter of less than one mile" is an accidental generalization--it may be true throughout the entire history of the universe, but the laws of nature do not forbid the existence of such a sphere (Weinert 1995, p. 18).
Consider another example: Suppose in the history of the universe no Tyrannosaurus rex is longer than 60 feet. According to the necessitarian, this is a purely contingent fact--it just happens to be the case that no T. rex exceeded 60 feet, but it wouldn't be physically impossible for a T. rex to have been a foot longer. By contrast, it would be physically impossible for a photon to have mass. The difference between the two cases is that the laws of nature do not forbid the existence of a T. rex exceeding 60 feet. On a regularity account, however, the laws of nature simply describe truths which hold true throughout the history of the universe. If no T. Rex exceeds 60 feet in the entire history of the universe, the regularity theorist seems forced to describe this truth as a law of nature. On a regularity account, there simply is no distinction between laws of nature and accidental generalizations.
How is all of this relevant to our final criterion for the natural? On a necessitarian account, natural laws describe what must be the case--they limit what is or is not possible. It would be more accurate (and more poignant for our purposes) to say that laws of nature limit what is possible assuming only natural causes are present. So if something occurs which is not possible according to laws of nature, then we have a genuine instance of a supernatural event--a physically impossible event. On a regularity account, however, there can be no violations of natural laws by definition because natural laws simply describe what happens in the whole history of the universe without invoking physical necessity or physical possibility.
If one rejects a necessitarian account of laws of nature in favor of a regularity theory, then the 'behaves in accordance with laws of nature' criterion for the natural will have to be abandoned--on a regularity theory every event that occurs in the history of the universe behaves in accordance with the laws of nature by definition. There simply cannot be a violation of a law of nature on a regularity account. On such an account every event which happens within nature obeys the laws of nature and thus, barring causal overdetermination, has natural causes. On a regularity account, the natural-supernatural distinction is abandoned. I will consider reasons why we should not abandon the natural-supernatural distinction in the next section.
Some laws of nature, such as statistical laws, are merely dispositions or propensities for natural objects to behave in certain ways under certain conditions. Such laws leave room for deviations from the normal course of behavior exhibited by the objects they govern. Nevertheless, the circumstances under which such deviations can occur is still constrained by statistical laws. For example, in quantum mechanics the spontaneous appearance of virtual particles out of a vacuum appears to violate the law of conservation of energy. However, the Heisenberg uncertainty principle allows for the creation of short-lived virtual particles from a vacuum on subatomic scales smaller than the radius of a proton. The creation of a lasting macroscopic physical object such as a chair out of nothing would constitute a violation of a law of nature.
Mental states also appear to be governed by laws of nature. If reductive physicalism is true, mental states obey the same laws of physics that any physical system must obey even if the laws of physics do not make explicit reference to any mental states. If either property dualism or interactionist substance dualism is true, there are laws of nature which govern correlations between mental states and brain states. These laws would not be laws of physics, but psychophysical laws governing, for example, how willing your arm to go up causes it to do so (if mental-to-physical causation occurs) or how being in a certain brain state causes you to feel pain (if physical-to-mental causation occurs).
One objection to identifying the natural with that which behaves in accordance with laws of nature is that the laws themselves, some of which are amenable to scientific investigation, do not behave in accordance with laws of nature. There are two independent issues here. First, scientific laws (our approximations to the laws of nature) are not strictly amenable to scientific investigation in the sense of being scientifically explicable. We cannot provide a scientific explanation for why matter or energy cannot be created or destroyed. We can only explain natural phenomena in terms of such laws. These laws are brute facts where all explanation must ultimately terminate. In the case of our most general laws of nature, we can use science to determine approximations to what these laws are, but not why they obtain rather than a different set of laws. Thus, like qualia appear to be, the laws of nature themselves are in principle scientifically inexplicable. This would provide even stronger grounds for rejecting the 'amenable in principle to scientific investigation' criterion for the natural if we wanted to countenance the laws of nature themselves as part of the natural realm. The second point raised by the objection is that the laws of nature themselves do not obey laws of nature. But this makes our present criterion self-refuting only if the laws of nature are countenanced as part of the natural world. Perhaps they belong to the nonnatural realm. Although laws of nature may necessitate the behavior of natural objects, since by definition they are unchanging they are not causes of change in the behavior of natural objects. Since natural laws cannot be located in space and time and are not causes of change, they appear to qualify as nonnatural abstract objects.
Our central goal in introducing the idea that the natural is that which behaves in accordance with laws of nature was to resolve a difficulty with the 'amenable in principle to scientific investigation' criterion for the natural. That difficulty was that it appears impossible to say a priori whether some phenomenon is amenable to scientific investigation. Only an a posteriori answer is available: In order to determine whether a phenomenon is amenable to scientific investigation we have to subject it to a scientific investigation. If we arrive at a successful scientific explanation of the phenomenon, we have determined that it is amenable to scientific investigation. But for phenomena which continue to resist scientific explanation, we cannot say categorically that they cannot be explained in principle because we have no way of knowing that. There is simply too much room for mistaking the limitations of our cognitive resources or our empirical access to a phenomenon for a metaphysical truth.
The idea that the natural is that which obeys natural laws can explain how something can be amenable to scientific investigation in principle in terms of a more basic property; but it does so at the expense of raising further difficulties associated with the concept of natural laws and does not shed light on the meaning of 'natural' in practice. If the regularity theorist is right, we cannot appeal to the concept of physical necessity to explain events in the natural world. Moreover, even if we assume a necessitarian account of laws of nature, we cannot (from our limited point of view) declare what those laws are with full confidence. Our scientific laws, after all, are merely approximations of genuine laws of nature. They are subject to qualification under different circumstances and we can never exhaust the nearly limitless circumstances under which our scientific laws may need to be qualified. Since no two events are entirely identical, what appears to be a violation of a law of nature may turn out to be, upon further investigation, a new circumstance where our scientific laws need to be modified.
How can we have knowledge of the genuine laws of nature, as opposed to approximations to them? Essentially we can only have knowledge of scientific laws, not the laws of nature themselves, through the empirical discovery of regularities in nature. But if we cannot have knowledge of the laws of nature themselves we cannot say with full confidence that any event is truly a violation of a law of nature (as opposed to a violation of a scientific law). We seem to have reached an impasse in our analysis of the meaning of the term 'natural'. None of our criteria can tell us how to draw the natural-supernatural distinction in practice.
Where does this survey of criterion for the natural leave us? We have seen that there are good reasons for rejecting the simple identification of the natural with the physical, with the spatiotemporal, or with that which is amenable in principle to scientific investigation. In theory, the natural can be identified with the physical or supervenient upon the physical and with that which behaves in accordance with laws of nature. These two criteria can be combined as jointly necessary and sufficient theoretical conditions for the natural. Anything appropriately labeled as natural would have to meet both conditions. Being strictly physical, spatiotemporal, or amenable in principle to scientific investigation would be sufficient conditions (but not necessary ones) for something to be natural. In other words, anything that was physical, spatiotemporal, or scientifically explicable would clearly be natural, but these are not requirements something must meet in order to be considered natural. Conversely, however, being nonphysical, nonspatiotemporal, and scientifically inexplicable would be necessary but not sufficient conditions for the nonnatural--anything that is physical, spatiotemporal, or scientifically explicable cannot be nonnatural.
Combining these criteria with a negation of our remaining criteria for the natural, we are left with the following jointly necessary and sufficient theoretical conditions for the nonnatural: (1) it is nonphysical and not supervenient upon the physical; (2) it is nonspatiotemporal; (3) it is scientifically inexplicable in principle; and (4) it fails to behave in accordance with natural laws. Abstract objects, as paradigm cases of nonnatural objects, meet all of these conditions. Since the supernatural is a more specific subclass of the nonnatural restricted to nonnatural causes of events within nature, a supernatural event would have to be an event in the natural world which has a cause meeting all four of the conditions given above. In theory, causes of events in the natural world which meet these criteria are supernatural causes. But, as we have seen, it is impossible to apply any of these theoretical criteria in practice because of our limited epistemological resources in answering metaphysical questions. We simply do not know what sorts of objects, properties, or phenomena count as nonphysical, what sorts resist scientific investigation in principle, or which actually behave in accordance with genuine laws of nature. Does this mean that we should abandon the natural-supernatural distinction?
Given that we can formulate self-consistent jointly necessary and sufficient theoretical conditions for both the natural and supernatural, the natural-supernatural distinction can be coherently spelled out in theory. However, this does not mean that the distinction has any validity in practice. Labeling an occurrence a supernatural event implies that the "the power and laws of nature could not bring [that event] about" (Hepburn 1972, p. 454). However, the objection goes, we are not in the epistemological position to make such a determination. As Ronald Hepburn points out,
It is anything but easy... to elaborate coherently the nature-supernature distinction. Crucial to it is the claim that we can distinguish what lies within the capacities of nature from what lies beyond them. Our knowledge of nature's powers and laws is itself derived from our experience and observation of events. What we judge to be possible depends upon what we have reason to believe actually occurs or has occurred... [W]hat shall we do with the happenings that, eventually, we wish to label miraculous? To exclude them would be to imply that we already know what nature's powers are, that there are criteria prior to experience by which we interpret our observations. But to include them makes it impossible for us to treat them later as miraculous exceptions to natural laws (Hepburn 1972, p. 454).
Hepburn points out a crucial dilemma: Either we retain the natural-supernatural distinction in practice, placing ourselves in the difficult position of trying to determine a priori which sorts of phenomena are physically impossible--something we cannot say with full confidence--or we abandon the practical distinction altogether, thereby making 'naturally-caused event', for all practical purposes, equivalent to 'any event which occurs in the natural world'. Given these two forced options, we should opt for retaining the natural-supernatural distinction in practice simply because the cost of abandoning it is too great.
Naturalism would become trivially true if we abandoned the practical natural-supernatural distinction. The argument for trivial naturalism would be that it is impossible to draw the natural-supernatural distinction in practice and therefore we are never justified in positing supernatural causes because we can never say that the cause of an event is not natural. This implies that naturalism always wins by default on epistemological grounds, even though not necessarily so on metaphysical grounds. Naturalism becomes trivially true because no matter what phenomenon we encounter, we cannot say it is not a naturally-caused phenomenon. If we abandon the natural-supernatural distinction in practice, so far as we can tell any logically possible event may be a naturally-caused event. Consequently, naturalism becomes trivially true in practice but uninformative and supernaturalism is never an option for belief since all causes of events in the natural world may be natural causes.
Hepburn's point--that we simply do not know what is within nature's powers or laws--is borne out in every impasse we have hit in trying to determine how to apply our theoretical criteria for the natural and the supernatural in practice. While we have a pretty good idea of what 'natural' means in theory, it is difficult to say in practice what counts as instance of an event with a natural or supernatural cause. Herbert Spiegelberg argues that because the supernatural (as a species of the nonnatural) is defined in wholly negative terms, it has little--if any--positive content. If by a supernatural event we mean not merely an extraordinary event but an event that is beyond the power of nature to produce, requiring a suspension or violation of the laws of nature, Spiegelberg points out:
Now it takes very little to see that even this conception of a supernatural event is predominantly negative. It implicitly denies that things usually happen this way and that we, with our knowledge of nature, can ever hope to comprehend them. But we learn nothing positive about the nature of the miraculous event... [W]e are faced with a merely negative understanding of the supernatural (Spiegelberg 1951, p. 348).
But what does a negative understanding of a concept imply? Spiegelberg notes that negative concepts refer to the absence of specific characteristics. For example, the term 'nonwhite' refers to all colored things except those which are white. While 'nonwhite' is a negative concept, it indirectly refers to the positive denotative complement of the term white--colored things. Because 'white' is a species of the proximate genus of colored things, nonwhite too is a species of colored things. The problem Spiegelberg sees with the concept of the supernatural is that there is no proximate genus to which both the natural and nonnatural refer:
[I]n an example like 'not-white'... after subtracting white, all the other known colors remain as, what one might call, the denotative complement. But what exactly is the denotative complement in the case of the not-natural, once we have subtracted the whole range of natural objects and events? It is at this point that the heart of the difficulty appears: we cannot point out a denotative complement to the name 'natural' (Spiegelberg 1951, p. 354).
Spiegelberg argues that because we literally know nothing about the nonnatural we cannot find any positive attributes common to both the natural and nonnatural (Spiegelberg 1951, p. 354). While denying that supernatural terms are completely meaningless, Spiegelberg concludes that "they have no more meaning than an expression like 'the 100th dimension' or '100-valued logic'" (Spiegelberg 1951, p. 355).
Despite this conclusion, Spiegelberg concedes that supernatural terms may have what he calls diagnostic meaning. Diagnostic meaning refers to identifying criteria "for recognizing cases of the supernatural when we see them and for deciding whether a specific pretender to the supernatural title has any right to it" (Spiegelberg 1951, p. 355). However, Spiegelberg is skeptical that the wholly negative concept of the supernatural is meaningful enough to provide means for the reliable identification of the supernatural:
As long as we are unable to point out as much as one positive complement to all the natural properties excluded by the supernaturalist we would never have a chance to tell with assurance: 'This event must be a case of the supernatural... an intercession from a world outside nature into nature' (Spiegelberg 1951, p. 356).
I agree that once we subtract out the natural, there is no positively characterized ontological category to which the nonnatural or supernatural belongs. This does make it impossible to positively confirm that an event had a supernatural cause in the way that we can confirm that engine failure caused a plane to crash. However, this does not strip the supernatural of diagnostic meaning altogether. All that is needed for diagnostic meaning for the supernatural is some reasonable criteria for what a supernatural event would look like. We do not need to say that something meeting those criteria must be supernatural; we need to say only that a supernatural event must meet those criteria. This does, of course, leave open the possibility that something entirely natural could also meet those criteria. But, as we shall see, this concession is not necessarily fatal to drawing out the natural-supernatural distinction in practice. It allows us to distinguish a class of clearly naturally-caused events from a class of possibly supernatural events. While this does not provide a definitive classification for the supernatural, it does narrow down our possible candidates for a supernatural event.
Spiegelberg gives two reasons for denying that we can secure indirect evidence for the supernatural in the way that we can secure indirect evidence for electrons. First, we can only devise testable predictions from well-defined positive concepts (Spiegelberg 1951, p. 356). Second, it is impossible to confirm conclusively that any event has a supernatural cause: "The natural signs predicted as indications of the supernatural are always so indefinite that they can never constitute conclusive proof either way. No confirmation is possible, since no disconfirmation can be conceived of that would unmistakably settle the case" (Spiegelberg 1951, p. 356).
Consider Spiegelberg's second reason for denying that we can secure indirect evidence for the supernatural. I agree that we cannot provide absolutely certain evidence for the supernatural. However, I contend that, in principle, we could provide evidence that any reasonable person would regard as conclusive evidence of the supernatural. First, we can provide conclusive evidence that a certain event occurred. The evidence that an event occurred will clearly be of a different sort than experimental evidence for a scientific hypothesis. The evidence for a scientific hypothesis often consists of repeatable results obtained under carefully controlled conditions, whereas the evidence that any sort of past event occurred is not going to be repeatable on demand in this way. Nevertheless, the evidence that an event occurred can be strong enough that it would be irrational to deny that the event occurred. For example, we have very reliable evidence that a nuclear bomb fell on Hiroshima in 1945, evidence that goes beyond even simple historical records, such as eyewitness testimony, video footage, radioactive traces, etc.
Second, we can provide very good reasons for denying that a certain kind of event could plausibly be construed as an event with a natural cause. Some imaginable events, if they occurred, would be so suggestive of a supernatural cause that it would be unreasonable to hold out indefinitely for a natural explanation of them. I will defend this point in more detail when I present my criteria for drawing out the natural-supernatural distinction in practice. Contrary to Spiegelberg's assertion, there is no reason in principle why there couldn't be conclusive evidence for an event extremely suggestive of a supernatural cause. Moreover, even if we could say only that an event was suggestive of a supernatural cause, though not conclusively so, Spiegelberg is wrong to assert that disconfirmations are not possible. A successful natural explanation of an allegedly supernatural event would be as much disconfirmation of a supernatural cause as the evidence for evolution is disconfirmation of six-day creationism.
Spiegelberg's first reason for denying that we can secure indirect evidence for the supernatural--that we cannot derive testable predictions from supernatural explanations--is related to the following objection: "By what method, if any, can we expect to identify an instance of [supernatural causation] in reality? Without such a method we would be cut off from any possibility of genuine knowledge" (Spiegelberg 1951, p. 357). Spiegelberg's point is that, unlike successful naturalistic explanations grounded in scientific method, we have no methodology on which to ground supernatural explanations.
While I agree with Spiegelberg that, unlike natural explanations, we have no grounds for evaluating competing supernatural explanations for an event, this does not mean that we cannot identify a likely candidate for a supernatural event. I contend that, in principle, there cannot be any methodology which would provide reliable supernatural explanations of an event because of the nature of supernatural causation. Supernatural causation requires a supernatural cause of a natural effect. But, by definition, we cannot have empirical access to a supernatural cause. We can have empirical access only to the effect of a supernatural cause. In order to provide reliable explanations of phenomena we must establish causal relationships through scientific investigation. But since we have no empirical means for gaining information about supernatural causes themselves, we cannot establish laws which express causal relationships between supernatural causes and natural effects, should there be any. We need access to both cause and effect to establish any sort of reliable causal explanation. However, this means only that we cannot understand how a supernatural event occurred; it does not mean that we cannot say that it did occur.
Spiegelberg maintains that ordinarily we establish the existence of an instance of a negative concept by establishing the existence of part of its positive denotative complement--e.g. to establish the existence of 'not-white' we establish the existence of some other color. But since we can find no positive denotative complement to the nonnatural or supernatural, it will be extremely difficult to establish an instance of it:
Since we found that the supernatural in its prevalent negative interpretation has no such positive denotational complement, this simple method of verification is out of the question. Suppose, then, some object should present itself as a candidate to supernatural dignity... All we can hope to do, if we want to check such a claim critically, is to show that it does not fit into any known category of nature. We have therefore to make sure of a comprehensive review of the natural realm, running through all its known and conceivable categories with a view to establishing at least indirectly that here is unmistakable evidence of something different from the familiar pattern and not covered by our 'natural' categories. To do this exhaustively may not seem impossible on principle. But on any concrete occasion this is certainly no small assignment (Spiegelberg 1951, p. 358).
Although establishing an instance of supernatural causation may not be impossible in principle, Spiegelberg contends that to do so in any particular case one would have to eliminate all possible natural explanations for an event--including scientific explanations cast in terms of a future science not yet available to us. Of course, this is not a practical possibility. But even if we could somehow pull this off, Spiegelberg further argues that appeals to the supernatural in such cases, given the negative character of the supernatural, simply reduce to appeals to ignorance.
Given the lack of any methodology for evaluating competing supernatural explanations and the lack of any empirically compelling claim to revelation, I agree that the most we could say in such cases is that the cause of the event in question is not natural. We couldn't say anything about the nature of the supernatural cause itself. In this sense any particular supernatural explanation would be an appeal to ignorance. However, when no natural cause for an event can be found after a completely exhaustive search for natural causes, our only apparent recourse is to postulate a supernatural cause for the event. In actual cases of a possible supernatural event, of course, we will not be able to conduct a completely exhaustive search for natural causes. We cannot do this even for events with natural causes. Other considerations I will consider briefly will make it clear that Spiegelberg's requirement of an exhaustive search of the natural world to establish that a supernatural cause of an event is far more likely than a natural one is too strong.
Spiegelberg's fundamental claim is that in order to say that a phenomenon is supernatural with assurance we must have an exhaustive knowledge of the natural realm and what is possible within it:
There is, however, a much more presumptuous claim involved in any serious affirmation of the supernatural. The supernatural interpretation presupposes a knowledge of the natural sphere so clear and so exhaustive that we can be absolutely sure it does not offer any room for the phenomenon under consideration... Whoever asserts that a certain event is supernatural also asserts that he has complete knowledge of the field marked off as nature as well as full knowledge of what is and what is not possible within it (Spiegelberg 1951, p. 358).
Do we actually have to have complete knowledge of the natural world in order to distinguish between an event with a natural cause and an event with a supernatural one? If we want complete assurance that an event has a supernatural cause we may need an exhaustive survey of the natural world. However, Spiegelberg does not address the possibility that we could determine that an event has a supernatural cause with a reasonable degree of confidence. I will now defend the position that we are in a position to identify events with supernatural causes with a reasonable degree of confidence.
It is difficult, but not impossible, to draw the natural-supernatural distinction in practice. Our impasse in determining what counts as an event with a supernatural cause does not reflect a deficiency in our theoretical criteria for the supernatural, but rather the practical limitations of our epistemological access to fundamental metaphysical categories like 'physical', 'laws of nature', etc. We simply cannot say categorically what it means for something to be physical, to be amenable in principle to scientific investigation, or to be a law of nature. This is a practical difficulty that can be overcome by replacing our theoretical criteria for the supernatural with our closest approximations to them in practice. Thus, what we need is a less theoretical set of conditions for events which may have supernatural causes--what I will call criteria for a likely candidate for a supernatural event. Given that we are working with approximations to our theoretical criteria, it is possible, though unlikely, that something considered entirely natural in terms of our theoretical criteria would be consistent with our practical criteria for a likely candidate for a supernatural event. In other words, anything meeting these practical criteria could be entirely natural, but it is unlikely that anything meeting all of them would be. What would these in practice criteria look like?
In order to answer this question, we must ask another: What would be an indication that an event has a supernatural cause? Consider three examples of what many people would regard as clear-cut supernatural events--stars lining up to spell out 'God exists', the resurrection of a corpse after a century of decomposition, and a marble statue waving its hand at you. What is it about these examples that makes us think these events cannot have solely natural causes? Is it simply that they do not ordinarily happen, much like extinction-level meteor impacts with the Earth? Or is it something more?
In addition to meeting approximations of our theoretical criteria for the supernatural, these examples seem to display other commonalities. Our theoretical criteria for the nonnatural consisted of the following jointly necessary and sufficient conditions--the nonnatural must be nonphysical and not supervenient upon the physical, be nonspatiotemporal, be scientifically inexplicable in principle, and fail to behave in accordance with natural laws. Our practical approximations to these theoretical criteria combined with the commonalties in the 'clear-cut' examples provide the following jointly necessary and sufficient conditions for a likely candidate for a supernatural event: (1) the cause of the event cannot be identified as any known physical force or entity nor is it supervenient upon any known physical force or entity; (2) the cause of the event cannot be located in space and time; (3) the event defies all attempted scientific explanations thus far; (4) the event appears to violate well-established scientific laws (as distinguished from genuine laws of nature); (5) the event is highly improbable if it solely has known natural causes; and (6) the event exhibits apparently purposive or intelligent behavior. Any event meeting these criteria is a likely candidate for a supernatural event. Note that being a likely candidate for a supernatural event is a necessary but not a sufficient condition for actually being a supernatural event.
The first three conditions for a likely candidate for a supernatural event seem relatively straightforward. The fourth and fifth conditions are more complicated and related in fundamental way. Note that it would be a stronger approximation to our theoretical criteria to say that a likely candidate for the supernatural violates scientific laws rather than appears to violate scientific laws. However, even given known scientific laws, it isn't always clear what constitutes a violation of a scientific law. In theory, an event with a supernatural cause really would be physically impossible--given fixed initial conditions, genuine laws of nature, and only natural causal factors, a physical system would have to behave in a certain way. This is true even given physical indeterminism--some outcomes will be more probable than others, but some outcomes will be excluded altogether. For example, while the Heisenberg uncertainty principle allows the creation of short-lived subatomic particles out of nothing, the creation of a macroscopic physical object out of nothing would be a violation of a law of nature. A physically impossible event would be an event which behaved contrary to what would be predicted given initial conditions, genuine natural laws, and all relevant natural causal factors. Because we often cannot know the initial conditions and relevant natural causal factors involved in bringing about an event even assuming that scientific laws are genuine laws of nature, a physically impossible event may not involve a straightforward violation of a scientific law.
For example, a resurrection can be described as a localized reversal of entropy, something which is not explicitly ruled out by scientific laws. However, given likely initial conditions (e.g. an advanced state of decomposition) and all likely relevant natural factors (e.g. the destruction of most of the physical information about the body as it was before death due to decomposition and the lack of an advanced replication machine), some events may be so improbable that the chances of such events occurring even once given our background knowledge of the relevant natural causal factors produces astronomical odds. Consider Richard Dawkins' example of a statue waving its hand at you:
In the case of the marble statue, molecules in solid marble are continuously jostling against one another in random directions. The jostlings of the different molecules cancel one another out, so the whole hand of the statue stays still. But if, by sheer coincidence, all the molecules just happened to move in the same direction at the same moment, the hand would move. If they then all reversed direction at the same moment the hand would move back. In this way it is possible for a marble statue to wave at us. It could happen. The odds against such a coincidence are unimaginably great... The number is so large that the entire age of the universe so far is too short a time to write out all the noughts! It is theoretically possible for a cow to jump over the moon with something like the same improbability (Dawkins 1987, pp. 159-160).
Dawkins argues that if such an improbable event occurred we should regard it as a supernatural event "because all our experience and knowledge tells us that marble doesn't behave like that" (Dawkins 1987, p. 159). These events do not straightforwardly involve violations of scientific laws because, in practice, all relevant natural causal factors are not given. If they were all known, however, then a resurrection would almost certainly involve a violation of scientific laws. Because of the limitations of our knowledge, we will have to work with likely initial conditions and likely natural causal factors. Certain sets of causal factors will be more likely to obtain than others. But because we do not know that a scientific law necessarily is violated (e.g. a levitation could result from some other natural force counteracting the law of gravity, such as magnetic repulsion), for all practical purposes, an event appearing to violate scientific laws reduces to a highly improbable event given only known kinds of natural causes. Note that an event which is highly improbable given known natural causes is not necessarily an infrequent event. If supernatural causes are present, we would expect a greater frequency of events which would be very unlikely if only natural causes were present. In order for this distinction to hold, we must assume something like a propensity interpretation of probability instead of a relative frequency interpretation--that is, we must assume that the probability of an event refers to a propensity of a physical system to produce a given outcome.
The last condition for a likely candidate for a supernatural event--that the event exhibits apparently purposive or intelligent behavior--is fairly ambiguous. Note that this final condition is the only condition which cannot ultimately be derived from our theoretical criteria for the nonnatural. Purposive or intelligent behavior contrasts with the way natural phenomena are often characterized--as impersonal or the result of impersonal, unintelligent causes. Natural forces are typically characterized this way because the basic natural causes which constitute the subject matter of physics are impersonal and unintelligent and physics has given us the most reliable conclusions of all the sciences. Of course this condition cannot be derived from our theoretical conditions for the natural or supernatural precisely because some natural objects like animals or even heat-seeking missiles exhibit purposeful behavior. Purposeful or intelligent behavior is not necessarily a sign of supernatural causation because natural objects also exhibit it. But our consideration of the three examples of 'clear-cut' supernatural events all included this element and I think we can tentatively conclude that no supernatural event would have impersonal causes and thus purposeful behavior should be one of the conditions for a likely candidate for a supernatural event.
We can elaborate on what is required for an event to meet the final condition by providing examples of events which do meet it. However, to back up the claim that it is unlikely for an event with a natural cause to meet all of the criteria I have outlined for a likely candidate for a supernatural event, I will consider an example of an event meeting all six conditions. Imagine that a religious leader claimed that he would return to vindicate his followers' faith that he is the savior of the world a half century after his death. Imagine that this leader made arrangements with his followers that his corpse would be kept on display after his death. Anticipating his resurrection and with vast financial resources, the followers ensured that there would be a substantial media presence at the displayed tomb fifty years later to the day. While the journalists reluctantly appeared at the site, fulfilling a contract, none of them seriously believed that this resurrection would occur. The leader's followers also ensured that a DNA test would be performed on the spot that day to confirm the identity of the corpse.
An hour after the DNA test results confirmed the corpse's identity, and while television cameras rolled, the corpse spontaneously transformed into a likeness of the original man. The likeness of the leader then turned to the astonished reporters and cameramen and said "I will bring peace and harmony to the world and heal the sick". At the moment those words were spoken, every human being on the planet lost all symptoms of disease. The likeness of the leader then said "I will return in 6 months" and disappeared. An extensive worldwide UN-sponsored medical check-up confirmed that no examined individual on the planet had any trace of disease.
This would clearly be a likely candidate for a supernatural event. The cause of the transformation and universal healing could not be identified as any known physical or spatiotemporal cause, would be scientifically inexplicable, would appear to violate natural laws, would be highly improbable given only known kinds of natural causes, and the transformed corpse's communication and apparent healing powers would exhibit purposeful behavior. I contend that in the hypothetical example just given the well-documented events could not plausibly be regarded as events with solely natural causes.
Throughout human history, supernatural causes have been invoked to explain droughts, earthquakes, thunderstorms, comets, the spread of disease, mental illnesses, mystical experiences, the orbits of the planets, the origin of living things, and the origin of the world, among many other phenomena. As the scientific revolution of the 17th and 18th centuries flourished, appeals to supernatural causation ultimately gave way to successful scientific explanations of various phenomena in terms of natural causes. Ever since its inception, science has increasingly strengthened the plausibility of naturalism by providing informative accounts of a wide range of phenomena in terms of natural causes. The more science has progressed, the less room there has been for postulating supernatural causes within a scientific account of the world, and if past experience is any guide, the trend will continue well into the future. This trend has led many to conclude that there probably are no genuine instances of supernatural causation. As science explains more of the natural world around us, appeals to supernatural causation become less plausible.
Many philosophers and scientists have concluded that the best explanation for our ability to develop successful scientific explanations for such a wide range of phenomena in terms of natural causes is that there are no genuine instances of supernatural causation. Barbara Forrest, for example, describes naturalism as "a generalization of the cumulative results of scientific inquiry" (Forrest 2000, p. 19). In other words, the best explanation for the success of science is that naturalism is true. Given the proliferation of successful scientific explanations for phenomena, Forrest concludes that there is "an asymptotic decrease in the existential possibility of the supernatural to the point at which it is wholly negligible" (Forrest 2000, p. 25). If naturalism were false, there would be some phenomena that could not be explained solely in terms of natural causes. However, because science can explain all of the uncontroversial phenomena we have encountered in terms of natural causes, there probably are no phenomena which cannot be explained in terms of natural causes. Therefore, naturalism is probably true.
This success of science argument rests on a crucial inductive premise--that we can infer that all phenomena can be explained in terms of natural causes from the ability of science to explain all of the uncontroversial phenomena we have encountered in terms of natural causes. Even if we accept the validity of this inductive inference, we still have to establish that all the uncontroversial phenomena we have encountered so far can be explained scientifically. Since there certainly are uncontroversial phenomena for which we lack successful scientific explanations--consider the prevalent gravitational influence of some unknown form of dark matter in the universe--I will defend a related but stronger argument for naturalism. This argument does not require us to have a successful scientific explanation for all well-established events in order to provide evidential support for naturalism.
A likely candidate for a supernatural event is not necessarily the result of supernatural causation given that meeting the criteria for a likely candidate is a necessary but not a sufficient condition for actually being a supernatural event. Thus, if naturalism is true, it does not necessarily follow that there will be no likely candidates for a supernatural event--it is possible, however unlikely, that a naturally-caused event would also meet the requirements for a likely candidate for a supernatural event. For example, suppose that a subject can induce out-of-body experiences at will in a laboratory setting. During several experimental trials, after this subject has induced an out-of-body experience, infrared cameras capture the outline of a person moving toward a bell which begins to ring in a room adjacent to the location of the subject's normal physical body. If such events occurred today, they would meet all of the criteria for a likely candidate for a supernatural event. Nevertheless, such events might be the result of entirely natural causes which could be understood only in terms of some future science not yet available to us. For example, one might postulate that human organisms possess natural astral bodies made of some unknown form of exotic matter which can detach from normal physical bodies in certain circumstances. In the absence of successful scientific explanations for such phenomena, however, uncontroversial instances of likely candidates for a supernatural event would make supernaturalism more likely to be true than not relative to a background scientific picture lacking natural categories for such events.
Regardless of such possibilities, if there are any events within nature that have supernatural causes, these events will be likely candidates for a supernatural event. Thus, if naturalism is false, there will be events which are likely candidates for a supernatural event. Even without a definitive set of criteria for identifying a supernatural event, we can see the beginnings of an argument for naturalism:
(P1) If naturalism is false then there are events which are likely candidates for a supernatural event.Or, to put the argument in another form:
(P2) There are no events which are likely candidates for a supernatural event.
(C) Therefore, naturalism is not false (i.e. naturalism is true).
(P1) If there are no events which are likely candidates for a supernatural event then naturalism is true.
(P2) There are no events which are likely candidates for a supernatural event.
(C) Therefore, naturalism is true.
The argument above forms the basic foundation of my defense of naturalism. As stated above, it is too broad to be useful; the crucial second premise simply cannot be established in the absence of omniscience. However, we can modify this argument into a more practical lack of evidence argument:
(P1) If after an intensive search of the natural world scientists and historians have found no uncontroversial evidence for likely candidates for a supernatural event then naturalism is probably true.
(P2) After an intensive search of the natural world scientists and historians have found no uncontroversial evidence for likely candidates for a supernatural event.
(C) Therefore, naturalism is probably true.
The lack of evidence argument assumes that if supernatural causation does occur, prima facie we should have uncontroversial evidence for events which are likely candidates for a supernatural event. There is no reason in principle why the occurrence of such events could not be established conclusively. On the other hand, if supernatural causation does not occur, we should expect to find no uncontroversial evidence for a likely candidate for a supernatural event. If naturalism is true, we will not necessarily fail to find uncontroversial evidence for a likely candidate for a supernatural event. However, we probably will not find such evidence. In other words, if we do find uncontroversial evidence for a likely candidate for a supernatural event, it is more likely than not that supernatural causation does occur and thus that naturalism is false.
Now that I have laid the groundwork for a defense of naturalism based on the lack of uncontroversial evidence for events which would probably have supernatural causes if they occurred, it is time to elaborate upon and defend the premises of the argument. First, since I have already used the crucial phrase without defining it, I want to clarify what I mean by 'uncontroversial evidence'. Uncontroversial evidence is not necessarily replicable experimental evidence, although that would certainly qualify as uncontroversial evidence. By uncontroversial evidence for a proposition I simply mean evidence which would lead any reasonable person to conclude that the proposition is true. For example, we have uncontroversial evidence that slavery was prevalent in 19th century America, that the continents have drifted apart over hundreds of millions of years, that the evolution of species has occurred, and that light is a form of electromagnetic radiation. What these propositions have in common is that they are accepted by a consensus of the experts doing research within the relevant empirical subject matter. Uncontroversial evidence is evidence that generates consensus among the experts in the relevant field.
The most serious objection to the first premise is that by requiring uncontroversial evidence for a likely candidate for a supernatural event, the lack of evidence argument sets the evidential standards required to make supernaturalism more likely to be true than not too high. In other words, by requiring such strong evidence, the argument stacks the cards in the naturalist's favor, making his desired conclusion too easy to reach. The objection fails, however, because it is entirely appropriate to require uncontroversial evidence in such cases. A general heuristic within every empirical discipline is that extraordinary claims require extraordinary evidence. In practice, this heuristic is applied to claims which are improbable relative to a well-established nexus of scientific laws, theories, and facts, or to well-established historical accounts. For example, in 1989 Stanley Pons and Martin Fleischmann claimed that they had produced a significant nuclear fusion reaction in a laboratory at room temperature--a phenomenon dubbed 'cold fusion'. This contradicted the well-established scientific wisdom that significant fusion requires high temperatures in order for two nuclei to gain enough velocity to overcome the force of repulsion between them and fuse. Moreover, when other scientists performed the same experiment they found no measurable generation of energy (Stenger 1990, pp. 66-67).
Cold fusion was improbable because it conflicted with our well-grounded knowledge of what is required for nuclear fusion reactions and the inability of other researchers to replicate the phenomenon supported this assessment. That any measurable fusion reaction could occur at room temperature was an extraordinary claim that the scientific community ultimately rejected in the absence of extraordinary evidence. What is poignant about this case for our purposes is that the processes alleged to be involved--however extraordinary or improbable--were entirely natural. By definition, a likely candidate for a supernatural event would be much more improbable (assuming known natural causes) than cold fusion or any other clearly natural process. It would occupy the furthest extreme on a continuum of improbable events. Since it is appropriate to require extraordinary evidence in order to establish the reality of clearly natural phenomena, it is certainly appropriate to require extraordinary evidence for the far more improbable likely candidates for a supernatural event. The highest evidential standards should be met before entertaining such possibilities.
Another important objection to the first premise is that supernatural causation may occur but be rare enough that we have not had the opportunity to gather any uncontroversial evidence for instances of a likely candidate for a supernatural event. It is possible that human beings have encountered likely candidates for a supernatural event, but such events have occurred so infrequently--perhaps once in a lifetime or less often--that we have not had the opportunity to substantiate them to a degree that would satisfy every reasonable person. Although I concede that this is possible, without substantial evidence for such candidates we would have inadequate grounds for postulating their occurrence.
An opponent might object that we do have some grounds for postulating that likely candidates for a supernatural event occur, such as testimony for the occurrence of such events from reliable witnesses, though we do not have grounds which would satisfy the scientific community or a consensus of historians. While we certainly do have testimony for the occurrence of likely candidates for a supernatural event, there is a large body of literature in psychology documenting the unreliability of eyewitness testimony even in mundane cases such as car accidents. Some witnesses will intentionally invent or exaggerate accounts of unusual phenomena in order to gain publicity or out of a desire to convince others of the reality of such events (Wiseman 1996, p. 832). Even when witnesses sincerely believe that they are presenting an accurate account of an incident, perceptions are often distorted when events occur unexpectedly and last for a brief duration (Wiseman 1996, p. 829). Furthermore, when events invoke strong emotional reactions, eyewitnesses typically reconstruct their memories of them according to their own biases and expectations, unintentionally reporting incidents which never happened or omitting crucial clues to what really happened (Wiseman 1996, pp. 832-33). Richard Wiseman, the Koestler Chair of Parapsychology at Edinburgh University, concludes that
[T]here is considerable evidence to suggest that individuals' beliefs and expectations lead them to be unreliable witnesses of supposedly paranormal phenomena--seeing what they want to see rather than what actually occurred (Wiseman 1996, p. 830).
If eyewitness testimony is unreliable in ordinary cases involving car accidents or robberies, it certainly does not provide adequate grounds for establishing the reality of extraordinary events such as likely candidates for a supernatural event. Since most testimonial evidence will not meet the rigorous standards that should be met before accepting the reality of an extraordinary claim, without the kind of independent corroboration required for scientific or historical knowledge we have insufficient grounds for postulating the occurrence of likely candidates for a supernatural event.
Another objection to the first premise is that supernatural causation may occur but be so rare that human beings simply have not encountered any evidence for likely candidates for a supernatural event. While this is certainly possible, we would clearly have no grounds for postulating supernatural causation if it were true. If we have no evidence of any kind for the occurrence of likely candidates for a supernatural event, we have no reason to believe that supernatural causation occurs. Another heuristic within any empirical discipline is that one should make the fewest number of assumptions necessary to account for one's observations. This principle of parsimony--'Do not multiply entities beyond necessity'--is often called Ockham's razor.
J. J. C. Smart points out that, fundamentally, "Ockham's Razor counsels us against an unnecessary luxuriance of principles or laws or statements of existence" (Smart 1984, p. 118). Smart formulates the principle as "others things being equal, we should prefer more simple theories to more complex ones" (Smart 1984, p. 120). He emphasizes that simplicity considerations are only important when competing accounts are equally compatible with the evidence: "Ockham's Razor does not imply that we should accept simpler theories at all costs. The Razor is a method for deciding between two theories that equally account for the agreed-upon facts" (Smart 1984, p. 124). Given this basic heuristic, if we have no evidence for likely candidates for a supernatural event, we should adopt the simplest explanation for this fact--that only natural causes are operative within the natural world. If every caused event we have encountered can be explained in terms of natural causes, there is no reason to invoke supernatural causes that do no explanatory work for any particular events.
In actual scientific research it isn't always clear which of a host of explanations makes the fewest number of assumptions given that we never make explicit all of the assumptions that an explanation makes. Nevertheless, some accounts are clearly relatively simpler than others. A naturalistic account invoking only natural causes is simpler than one which, in addition, invokes supernatural causes. The interesting question for our purposes is whether natural causation alone can plausibly account for all the data. If there is no evidence for likely candidates for a supernatural event, then a naturalistic account of the world can plausibly account for all the data.
For all practical purposes the absence of uncontroversial evidence for an extraordinary claim is equivalent to the absence of any evidence at all for it. This is because we already have a great deal of well-supported evidence against any extraordinary claim by its nature. For example, we have a great deal of evidence against people being able to walk on water. If we can assume that a reasonable person should accept (at least tentatively) the more reliable source of evidence when weighing conflicting sources, the evidence we have against an extraordinary claim will always outweigh any testimonial evidence for it that would not satisfy the scientific community or a consensus of historians. However, uncontroversial evidence for an extraordinary claim could outweigh the evidence we have against it. Had other scientists been able to produce a measurable fusion reaction at room temperature, the proponents of cold fusion would have been vindicated. Events like these--often called scientific revolutions--are common in the history of science. For example, the Michelson-Morley experiment carried out in the late 19th century and replicated several times since provided uncontroversial evidence that light does not travel in a medium. Only in the presence of such uncontroversial evidence could one reasonably reject the view of a consensus of the experts researching that subject matter.
If we have no good evidence for likely candidates for a supernatural event, then we clearly have no reason to postulate supernatural causation. Refusing to accept the existence of supernatural causes, however, is not equivalent to rejecting their existence. Accepting naturalism requires a stronger commitment--a denial of the existence of supernatural causes. Ockham's razor provides us with probablistic grounds for (at least tentatively) denying the existence of supernatural causation rather than suspending judgment and leaving the possibility of it open. If we have no uncontroversial evidence for likely candidates for a supernatural event, the simplest explanation for this is that there are no supernatural causes influencing the natural world. Dubious eyewitness testimony, second-hand accounts, and other inadequate sources of evidence for likely candidates for a supernatural event could always be explained more simply than the actual occurrence of such events in terms of clearly natural causal factors such as deception, exaggeration, memory errors, perceptual errors, or misinterpretation.
The simplest explanations which can account for the data ultimately constitute all scientific and historical knowledge. But even if the simplest explanation is not always the best explanation , rejecting the existence of entities which are not only unnecessary but implausible given current scientific understanding is more reasonable than suspending judgment regarding the existence of such entities. This is why Smart ultimately concludes that "plausibility in the light of total science is the best touchstone of metaphysical truth" (Smart 1984, p. 128). Furthermore, as P. J. McGrath points out, if we are rationally compelled to suspend judgment rather than reject the truth of scientifically implausible universal existence claims for which we have no evidence (such as 'unicorns exist'), we are forced into skepticism about the possibility of scientific and historical knowledge altogether:
We are clearly in a particularly poor epistemic position with regard to the existence of Descartes's evil demon, since if he did exist, he would ensure that evidence for that fact is concealed from us. But if this implies that we do not know whether this evil demon exists or not, then we do not know whether or not we are constantly being deceived (McGrath 1987, p. 55).
Thus, insofar as we can be reasonably confident in accepting scientific and historical accounts in general, we can be reasonably confident in accepting the simplest explanation for the lack of uncontroversial evidence for likely candidates for a supernatural event--that supernatural causation does not occur.
Before considering the evidence for likely candidates for a supernatural event, I want to address a common objection to the lack of evidence argument regarding the relationship between science and naturalism. It is generally agreed upon that science presupposes methodological naturalism but not metaphysical naturalism. That is, science assumes that only natural causes are operative within the natural world for methodological purposes because scientists have no empirical access to any supernatural causes which may reside outside of the natural world. Methodological naturalism is a necessary assumption for successful scientific investigations because no scientific explanation could be formed for instances of supernatural causation. Science has to provide naturalistic explanations for events if it is to provide any explanations at all because we have no reliable empirical means for deciding between competing supernatural explanations. But science itself is not committed to the metaphysical thesis that, as a matter of fact, the natural world is a closed system where every caused event has a natural cause. Throughout this essay 'naturalism' has referred to this stronger metaphysical thesis rather than the relatively uncontroversial methodological strategy.
One might object that science could never falsify naturalism because scientific explanations are never cast in terms of supernatural causes. However, while scientific explanations are inherently naturalistic, scientific discoveries could strongly suggest that an event has occurred which could not plausibly be explained in terms of natural causes. For example, had human beings been the only life to appear on the planet Earth immediately after it was habitable, with no evidence of evolution from previous ancestors and no fossils of extinct species ever found, this would be a scientific discovery which would strongly suggest a supernatural cause of the origin of human beings. Science has undermined the credibility of all forms of supernaturalism not because science assumes that only natural causation occurs as a methodological principle but because science has been successful in using that assumption. There simply are no gaps in our scientific picture of the world which seem to require an appeal to supernatural causes. The simplest and most straightforward explanation for the success of methodological naturalism as a scientific strategy is that metaphysical naturalism is true.
Thus the plausibility of naturalism to so many academics does not rest primarily on philosophical presuppositions but on the stunning success of scientific explanations of the natural world. In particular, the success of science has extended into several areas where explanations in terms of supernatural causes have been invoked in the past. Most philosophers and scientists today simply find no need to appeal to supernatural causes in order to explain any phenomena within the natural world. Our scientific picture of the world developed quite independently of philosophical concerns is entirely consistent with naturalism. Specific scientific discoveries have strengthened the plausibility of naturalism insofar as they are fairly straightforwardly what one would expect to obtain if naturalism is true.
In order to establish that naturalism is probably true, we have to establish that the second premise of the lack of evidence argument is true. That is, we must establish that, as a matter of fact, we have no uncontroversial evidence for instances of a likely candidate for a supernatural event. Rather than attempting a daunting survey of the evidence for particular events, however, I will consider the overall state of the evidence from the most likely sources of evidence for likely candidates for a supernatural event. In particular, I will consider two potential sources of uncontroversial evidence for likely candidates for a supernatural event. First, I will consider whether there is any good evidence that supernatural causation is likely to have occurred in the distant past. This evidence will consist of a broad outline of the history of the universe from its origin to the present which draws on the findings of cosmology, geology, chemistry, biochemistry, molecular biology, evolutionary biology, paleontology, and physical anthropology. Then I will consider whether supernatural causation is likely to have occurred within human history. It is increasingly difficult to establish that an extraordinary event has occurred the farther back in human history we go given the paucity of historical evidence available for such events. Thus I will restrict my analysis of the evidence within human history to the recent past, given the preponderance of so many different and highly reliable means for establishing the occurrence of events of any kind given modern investigative techniques and technology. This evidence will consist of an assessment of the present scientific status of parapsychological research.
Given data from various sciences we have constructed a broad outline of the development of the universe from its origin in the Big Bang to the present. Cosmology has provided some of the details of the development of the universe immediately following the Big Bang. Physicists are reasonably confident that about 15 billion years ago the universe expanded from a singularity 10 billion billion times smaller than an atomic nucleus (Ronan 1991, p. 30). In an extremely brief period after the Big Bang--much less than a second--the fundamental physical forces of the universe were unified into a single superforce. In different stages also occurring well within a second, this superforce ultimately separated into the four fundamental forces operating today--gravity, the strong nuclear force, the weak nuclear force, and electromagnetism.
Gravity was the first force to split away, leaving a remnant of the superforce labeled the GUT (Grand Unified Theory) force. When the GUT force was beginning to split into the strong force and an electroweak remnant--in what cosmologists call inflation--a false vacuum in the early universe produced a repulsive force which caused the expansion of the universe to accelerate at a tremendous rate (Ronan 1991, p. 32). Simultaneously, slightly more abundant particles of matter interacted with antimatter, resulting in mutual annihilations which left radiation and remaining particles of matter behind (Ronan 1991, p. 34). Though inflation stopped when the separation of the strong and electroweak forces was complete, the universe continued its expansion. The electroweak force then separated into the weak force and electromagnetism, completing the separation of forces. Within 3 minutes after the Big Bang, protons and neutrons could combine to form the nuclei of atoms. 300,000 years after the Big Bang, the universe cooled down enough to allow electrons to combine with nuclei to form the first atomic matter, primarily hydrogen and helium atoms (Ronan 1991, p. 34).
While the universe continued to expand its temperature continued to drop. About 2 billion years after the Big Bang, gravitational attraction acting on an uneven distribution of matter caused hydrogen and helium gas to clump together into protogalactic clouds larger than today's galaxy clusters. In many cases, individual clusters of galaxies condensed out of a single protogalactic cloud. Within these clouds, regions of greater density condensed into protogalaxies. By 7 billion years after the Big Bang a large number of galaxies had emerged (Ronan 1991, p. 38).
Within the galaxies, gravitational attraction caused interstellar clouds of gas and dust to aggregate into disks of matter with a central bulge. Heat generated by the greater density of the central bulges resulted in the formation of protostars surrounded by disks of gas and dust. When the material in a central bulge reached a critical temperature of 10 million degrees Kelvin, fusion began and a star was born, sometimes surrounded by planets (Ronan 1991, p. 78). The most massive stars burned more quickly then the less massive stars, fusing elements as heavy as iron before exploding in a supernova, resulting in the formation of elements heavier than iron. Other interstellar clouds, such as the one that formed our solar system, would contain these heavier elements (Ronan 1991, p. 102).
Roughly 10 billion years after the Big Bang, about 4.6 billion years ago, the Sun formed out of an interstellar cloud of gas and dust. As the solar nebula surrounding the Sun cooled, different chemical substances condensed into small grains at various distances from the Sun. After about one thousand years these grains had coalesced into a rotating disc of planetoids. Multiple collisions in the early solar system fused the planetoids into larger protoplanets, which in turn fused into even larger planets. About 100 million years after the appearance of the planetoids the solar system had become relatively stable (Ronan 1991, p. 102).
By 4.5 billion years ago the Earth had emerged as a full planet. At this time large planetesimals were still impacting the Earth roughly once a month, blocking sunlight from reaching the Earth's surface (Hartmann and Miller 1991, pp. 34-35). Numerous high-speed impacts with the early Earth created an ocean of molten rock which was maintained for millions of years. Heavier elements such as iron sank into the Earth's core while the lighter elements rose to the surface to become part of the Earth's crust (Hartmann and Miller 1991, pp. 36-39). While most of an initial hydrogen-rich atmosphere leaked away into space, a secondary atmosphere was generated when carbon dioxide and water vapor escaped from the Earth's interior through volcanic eruptions (Hartmann and Miller 1991, pp. 65-66). Due to fewer impacts, the cooling crust began to solidify and water vapor condensed into rain, forming the Earth's oceans by about 4.4 billion years ago (Hartmann and Miller 1991, pp. 67-68).
Most biologists think that life originated from the Earth's primordial oceans about 4 billion years ago (Sagan 1980, p. 30). On the primordial soup model, lightning and the Sun's ultraviolet light (prevalent in the absence of an ozone layer) acted as energy sources which broke apart simple organic molecules in the atmosphere, allowing them to continually recombine into more complex molecules. As Carl Sagan explains,
The products of this early chemistry were dissolved in the oceans, forming a kind of organic soup of gradually increasing complexity, until one day, quite by accident, a molecule arose that was able to make crude copies of itself, using as building blocks other molecules in the soup (Sagan 1980, pp. 30-31).
Like the original self-replicating molecule itself, the copies of this replicator reproduced themselves. As long as the molecular raw materials were available, this resulted in a growing population of self-replicating molecules (Dawkins 1987, p. 129). The replicators probably acted as templates: "Smaller components fall together into the mould in such a way that a duplicate of the mould is made" (Dawkins 1987, p. 129).
The earliest forms of crude self-replicating molecules probably utilized a replication process that was particularly prone to errors (mutations). Any process of replication, in fact, will produce errors in the reproductions of the original (Dawkins 1987, pp. 128-129). This led to variations within the population of replicators. Some of the altered copies may have lost their ability to reproduce themselves entirely, others may have reproduced themselves less efficiently than their predecessors, and still others may have reproduced at the same rate as their predecessors. But occasionally an altered copy of a self-replicating molecule will reproduce itself more efficiently than its ancestors (Dawkins 1987, p. 130). Because it reproduces itself more efficiently than the other replicators, more copies of the more efficient variant will be produced, and inevitably it will become predominant among the varied population of self-replicating molecules. When the raw materials are recycled as the molecules break apart (the molecular equivalent of death), the more efficient replicators ultimately eliminate its less efficient predecessors.
The process described above is evolution by natural selection on a molecular level. Once self-replicating molecules emerged from the oceans, the stage was set for the further evolution and diversification of life on Earth: "As time went on, they got better at reproducing. Molecules with specialized functions eventually joined together, making a kind of molecular collective--the first cell" (Sagan 1980, p. 31). The same process, in turn, led to the development of more complex unicellular and multicellular organisms. Given any population of reproducing organisms, the random occurrence of mutations in offspring--either through errors in the copying process itself or due to external factors like exposure to chemicals or radiation--will lead to variation in the individuals within the population. The genetic change in the offspring can decrease the offspring's chances of survival and reproduction, can have no affect on those chances, or increase those chances. Over successive generations, the adaptive variations which increase an organism's chances of survival and reproduction will become dominant in the population, ultimately weeding out less successful organisms. When the environment changes, those organisms that happen to be better suited to the new environment will thrive while maladapted organisms dwindle or become extinct. In this sense the environment nonrandomly selects for random variations among a population of organisms. When this process continues over successive generations, cumulative natural selection results in the accidental emergence of new species and a move from simple to more complex organisms over time.
The earliest known fossilized organisms are spherical and cylindrical single-celled bacteria which appeared 3.5 billion years ago (Hartmann and Miller 1991, p. 85). These bacteria were primitive prokaryotic cells which lacked a central nucleus and specialized organelles with genetic material scattered throughout the cell. Early prokaryotes probably photosynthesized sugars by combining carbon dioxide and water with sunlight (Thompson 1998, p. 38). Roughly 3 billion years ago pillow-like colonies of blue-green algae called stromatolites appeared in shallow water environments (Hartmann and Miller 1991, pp. 101-102). By about 2.3 billion years ago these microscopic plants had gradually depleted much of the Earth's abundant supply of carbon dioxide, replacing it with the byproduct of photosynthesis--oxygen (Hartmann and Miller 1991, p. 104). Since carbon dioxide was a greenhouse gas which had kept most of the surface of the Earth above freezing, the gradual loss of carbon dioxide was probably responsible for the first known widespread glaciation in Earth's history about 2.3 billion years ago (Hartmann and Miller 1991, p. 105).
By 1.4 billion years ago the first fossilized eukaryotic cells with a membrane-enclosed nucleus centralizing the cell's genetic material appear (Thompson 1998, p. 40). These eukaryotes would later evolve into animals (Hartmann and Miller 1991, p. 103). At about the same time the first multicellular plants appear. Fossilized worm burrows reveal the appearance of multicellular animals 1 billion years ago (Thompson 1998, p. 40). Molecular clocks also suggest the first appearance of multicellular animals at this time (Smith and Szathmary 1999, p. 110). Sexual reproduction also may have evolved around this time, allowing genetic material to mix more efficiently and causing organisms with beneficial mutations to spread more rapidly (Hartmann and Miller 1991, p. 121).
It was not until about 700 million years ago, over 3 billion years since life first appeared on Earth, that large multicellular animals--soft-bodied creatures including coral-like sea pens and organisms similar to jellyfish, worms, and sponges--appeared in the oceans (Hartmann and Miller 1991, pp. 117-119). Many of these organisms were thin leaf-like animals that probably breathed oxygen through diffusion (Smith and Szathmary 1999, p. 110). Because these Ediacaran fauna lacked the internal machinery for transporting nutrients and gases that we find in the later Cambrian species, these organisms probably did not evolve into the Cambrian invertebrates (Leakey and Lewin 1995, p. 22). They may have been responsible for the first possible mass extinction in Earth history when 75% of the single-celled species constituting the stromatolites went extinct. In about 100 million years the Ediacaran animals also became extinct, apparently an evolutionary dead end leaving few if any descendents behind (Leakey and Lewin 1995, pp. 23-24).
In the Cambrian explosion 550 million years ago, a diversity of hard-shelled marine organisms, such as clam-like brachiopods and crab-like trilobites, had evolved (Hartmann and Miller 1991, p. 118). During this period all of the major body plans found in living creatures today appeared within a few million years (Leakey and Lewin 1995, p. 16). The large proportion of atmospheric oxygen generated by photosynthesis by this time may explain the late arrival of diverse multicellular animals--larger animals could not have survived in the oxygen-deficient environment of the primordial Earth (Leakey and Lewin 1995, p. 21). By 500 million years ago a Cambrian mass extinction event wiped out 75% of all trilobite families and half of sponge families (Thompson 1998, p. 44). At 450 million years ago, after 90% of the history of the Earth until today had unfolded, the first fishes--jawless fishes with head armor--appeared while primitive lichen-like plants began colonizing the previously lifeless land (Hartmann and Miller 1991, pp. 125-126). These fishes were the ancestors of all vertebrates (Thompson 1998, p. 47). Another mass extinction event--the Ordovician extinction--occurred 440 million years ago (Leakey and Lewin 1995, p. 45). Trilobites, sponges, brachiopods, and fishes were decimated (Thompson 1998, p. 47). Jawed fishes, including cartilaginous sharks and bony fishes, and the first forests of seedless plants, appeared by 400 million years ago (Hartmann and Miller 1991, pp. 125-126). Spiders, millipedes, scorpions, and wingless insects appeared at roughly the same time (Hartmann and Miller 1991, p. 142).
The first amphibians evolved from lobed-finned fishes roughly 360 million years ago (Hartmann and Miller 1991, p. 143). At about the same time a Devonian mass extinction event occurred which primarily decimated corals and trilobites (Thompson 1998, p. 53). At 300 million years ago egg-laying reptiles appeared (Hartmann and Miller 1991, p. 151). By 240 million years ago reptiles were proliferating on land (Hartmann and Miller 1991, p. 151). The largest well-established mass extinction known (though perhaps second to the extinction of the Ediacaran fauna)--the Permian extinction--occurred 225 million years ago (Leakey and Lewin 1995, p. 45). 96% of Permian species had disappeared (Thompson 1998, p. 60). Mammal-like reptiles were decimated (Leakey and Lewin 1995, p. 45).
Nonetheless, rodent-like mammals emerged about 220 million years ago (Hartmann and Miller 1991, p. 184). The Triassic mass extinction occurred about 210 million years ago (Leakey and Lewin 1995, p. 45). This event nearly eliminated amphibians and reptiles from the face of the Earth (Thompson 1998, p. 66). The dinosaurs diversified about 200 million years ago. Some of these dinosaurs evolved into birds (Hartmann and Miller 1991, pp. 153-54). By 65 million years ago a Cretaceous mass extinction event--the impact of a large meteorite with the Earth--wiped out 75% of existing species, including the dinosaurs, allowing small mammals to fill in the niches they left behind (Hartmann and Miller 1991, p. 158).
The first tree dwelling primates with an opposable thumb and stereoscopic vision--groups similar to lemurs--appeared about 55 million years ago. By 35 million years ago, monkeys and apes had evolved (Hartmann and Miller 1991, pp. 191-92). The earliest human-like primates--australopithecines--emerged about 4 million years ago. About 2.5 million years ago, when australopithecines were walking upright and using crude bone and stone tools, a more advanced tool-maker--Homo habilis--appeared. Homo erectus emerged about 1.6 million years ago and may have gone extinct by about 300,000 years ago. Homo sapiens appeared about 350,000 years ago. One type of Homo sapiens, the Neanderthals, went extinct roughly 35,000 years ago (Hartmann and Miller 1991, p. 193). Modern humans--Homo sapiens sapiens--appeared about 150,000 years ago (Leakey and Lewin 1992, p. 205).
J. P. Moreland dubs such an account the naturalist's 'Grand Story' (Moreland 1998, pp. 40-41). However, it is important to acknowledge that this broad outline of the history of the universe was not developed by naturalistic philosophers. Rather, it was developed by scientists piecing together empirical evidence in a wide variety of fields irrespective of their individual philosophical dispositions. Thus it would be less polemical and more accurate to describe this account as the scientists' Grand Story. This scientific account of origins was developed on empirical grounds independently of philosophical concerns such as whether or not naturalism is true. That this scientific account conforms so well to the naturalist's expectations is significant empirical support for naturalism.
While there are certainly issues to be resolved in the details, no part of this broad outline of the history of the universe requires us to admit even a reasonable likelihood of supernatural causation. Because every caused event in this broad outline can adequately be explained (at least by scientific standards) entirely in terms of impersonal natural causes, it is unlikely that unresolved problems regarding the details of this outline will require us to appeal to supernatural causes. Despite a comprehensive investigation of the distant past, scientists have found no hints of supernatural causation anywhere in the history of the universe. An informed scientific picture of the world indicates that the processes which shaped the development of the universe and produced the diversification of life on Earth were entirely natural.
That scientists can make such an assertion with reasonable confidence is remarkable empirical support for naturalism. There appears to be no room for supernatural causation anywhere in our broad outline of the history of the universe. As Richard Dawkins points out, "A universe with a supernatural presence would be a fundamentally and qualitatively different kind of universe from one without. The difference is, inescapably, a scientific difference" (Dawkins 1997, p. 399). If supernatural causation had occurred in the distant past, events would have unfolded differently than they would have if only natural causes were present. Any detectable instances of supernatural causation would have produced gaps in our scientific account where likely candidates for a supernatural event occurred. But scientists have only uncovered evidence for clearly natural causal influences in this broad outline. That there appear to be no such gaps in our account of the distant past--that is, no events which require any likely candidates for a supernatural event--implies that naturalism is true. After all, scientists could have discovered otherwise. Our account of the history of the universe could have revealed that intelligent life and the universe as it appears today emerged within minutes after the Big Bang, for example. Given no plausible natural explanation for such advanced complexity appearing immediately out of relative simplicity, naturalism would be in serious conflict with the broad outline of the history of the universe. While we can certainly imagine histories of possible universes where it would be unreasonable to deny the existence of genuine instances of supernatural causation, we do not appear to live in such a universe.
On a scientific picture of the world, intelligence and purpose have no role in the distant past. As Daniel Dennett puts it, "In the beginning, there were no reasons; there were only causes. Nothing had a purpose, nothing had so much as a function; there was no teleology in the world at all... There was nothing that had interests" (Dennett 1991, p. 173). But then self-replicating molecules emerged which had primitive interests in replicating themselves. This involved avoiding circumstances that would inhibit or prevent replication and seeking replication-friendly circumstances. With self-preservation comes boundaries between self and other, purposeful behavior which aids survival, and ultimately intelligence and agency (Dennett 1991, p. 173-174). On our best scientific accounts, agency only emerges accidentally through natural selection with the appearance of higher organisms such as human beings. A likely candidate for a supernatural event, however, is an event for which no natural cause can be found which exhibits intelligent or purposive behavior. A supernatural event thus requires the activity of a supernatural agent. According to a scientific picture of the world grounded in a variety of different sources of empirical evidence, however, agency has no role in the universe prior to the emergence of life and then only appears in clearly natural organisms.
If we take this scientific account of the distant past seriously, we are compelled to conclude that the emergence of intelligent life on Earth was driven by entirely impersonal natural processes, primarily natural selection. The forces responsible for the evolution of life on Earth are the very antithesis of what we would expect from a supernatural agency, as Richard Dawkins makes clear:
Natural selection, the blind, unconscious, automatic process which Darwin discovered, and which we now know is the explanation for the existence and apparently purposeful form of all life, has no purpose in mind... It does not plan for the future. It has no vision, no foresight, no sight at all (Dawkins 1987, p. 5).
Within the natural world, the existence of intelligence is an accident of blind evolutionary forces. Our broad outline of the history of the universe indicates that, as far as we can tell, supernatural intelligences simply have no causal role to play in the natural world: "The fossil record implies trial and error, an inability to anticipate the future, features inconsistent with an [intelligent] Designer" (Sagan 1980, p. 29). While we do find an overall progression from simplicity to increasing complexity, this advance was not inevitable, but contingent on chance environmental circumstances. Moreover, given the long delay between the Big Bang and the origin of life on Earth--some 11 billion years--it is doubtful that evolution was guided by a supernatural agent in order to produce life in general or intelligent life (including human beings) in particular.
Natural selection has not only diversified forms of life but also resulted in the extinction of the vast majority of species on planet Earth. As Richard Leakey points out, "99.9 percent of all species that have ever lived are extinct... [L]ife's grip on Earth is evidently more precarious than we might like to accept" (Leakey and Lewin 1995, p. 197). The same processes which lead to the emergence of life also give way to its extinction, both individually for specific organisms and collectively for species as a whole. Surely human beings will share the fate of all other organisms and succumb to extinction. In fact, if we extrapolate the future of human beings from the course of human evolution in the past, it is likely that human beings will become extinct within the next ten million years (Hartmann and Miller 1991, p. 232). Although various forms of life will likely continue long after the extinction of the human race, all life on the planet will vanish when the Earth is no longer hospitable for life. And though there may be intelligent life elsewhere in the universe, given all likely scenarios for the future of the universe, the entire universe will one day become inhospitable for life. By all indications, the long absence of sentience from the beginning of the universe until the emergence of higher life will be followed by a permanent absence of sentience in the universe after the extinction of all life. As Ernest Nagel has succinctly put it, "human destiny [is] an episode between two oblivions" (Nagel 1960, p. 496). While the universe may superficially appear designed with life in mind, the indisputable cycle of speciation and extinction shows that life is temporary and apparently does not serve any purpose for any supernatural agencies.
When we consider that the first human-like primates appeared around 4 million years ago, "we realize with a shock that this represents a mere 0.1 percent of the history of the Earth... In other words, the duration of humanity thus far is so short that our whole species could be lost in the noise of geologic time" (Hartmann and Miller 1991, p. 194). The brief duration of human life on the geological time scale does more than illustrate the insignificance of our species within Earth history, not to mention the history of the universe. It also increases the likelihood of finding probable instances of supernatural causation within the distant past. If supernatural causation has occurred within human history and its frequency today (whatever that may be) is representative of its frequency in the past, given how brief human history is relative to the history of the Earth, we should expect to find abundant evidence of supernatural causation in the distant past. That we do not see such evidence increases the plausibility of naturalism. In other words, if we can find no evidence of supernatural causation in the distant past, it is unlikely that instances of supernatural causation have occurred within human history.
One could concede that there is no good evidence for likely supernatural causation in the distant past yet still affirm the existence of supernatural causation. For example, a deist could maintain that a supernatural deity caused the Big Bang yet has not intervened in the universe since its creation. While nothing in our scientific account of the distant past is inconsistent with this possibility, we would have no reason to postulate a supernatural cause at the beginning of the universe in the absence of any established instances of likely candidates for a supernatural event. The Big Bang could have simply been an uncaused event, for example. Perhaps there is no good evidence for the supernatural in the distant past because supernatural causation has only occurred within human history. Supernatural agents may intervene in nature only to make their presence known to human beings, having no other active role in the natural world. Though this is possible, it does seem to implausibly elevate the significance of a single species above everything else in the universe. Or perhaps supernatural causation has occurred in the distant past, but is simply undetectable given the sort of limited evidence that is available to us for such events. For events that have occurred in the distant past we can only discern major changes in Earth history over millions of years, for example, not events that take place on a time scale of minutes, hours, or days. Even so, however, events occurring on smaller time scales in the distant past could produce detectable changes in Earth history (just consider the extinction event which wiped out the dinosaurs).
Similar explanations are consistent with the lack of evidence for likely candidates for a supernatural event in the present--that such events occur but we simply do not notice them, for example. While all of these are possibilities, an appeal to Ockham's razor is appropriate here. Virtually all forms of supernaturalism that people have actually subscribed to posit that supernatural agents have a significant rather than marginal role in both origins and in human affairs. In any case, our broad outline of the history of the universe does establish that we have no uncontroversial evidence for instances of a likely candidate for a supernatural event in the distant past. There appear to be no supernatural agencies involved anywhere in the history of the universe.
Before we determine the scientific status of parapsychology, we must pause to establish the relevance of parapsychological research to establishing uncontroversial evidence for the existence of likely candidates for a supernatural event within recent history. Parapsychology is the scientific study of paranormal or psi phenomena. Michael Stoeber distinguishes between three main types of paranormal phenomena: receptive-psi, expressive-psi, and otherworldly-psi (Stoeber 1996, pp. 1-2). Receptive-psi includes telepathy, the paranormal reception of information or influence from other minds; clairvoyance, the paranormal reception of information or influence from physical objects or events; and precognition and postcognition, the paranormal reception of information or influence about the future and the past, respectively. Expressive-psi is exhausted by various forms of psychokinesis, where a subject's mind directly influences a person, animal, or object. The most interesting sort of paranormal phenomena for our purposes is what Stoeber calls otherworldly-psi:
The field of study is further expanded and complicated by the inclusion of the possibilities of the influence of discarnate spirits or forms of disembodied existence in explaining certain paranormal phenomena. In such contexts, psi events would involve realities beyond or distinct from that of this natural world (Stoeber 1996, p. 2).
On Stoeber's division of the paranormal, otherworldly-psi implies the involvement of disembodied supernatural agencies in receptive or expressive paranormal phenomena, whereas anything falling within the other two categories does not. Among the phenomena exhibiting otherworldly-psi Stoeber lists mediumistic communications, apparitions, poltergeists, angels, possession, out-of-body experiences, near-death experiences, and past-life phenomena (Stoeber 1996, p. 2).
Paranormal phenomena seem closely related, if not identical, to likely candidates for a supernatural event. All three kinds of paranormal phenomena falling within Stoeber's classificatory scheme meet at least the first five of our six criteria for a likely candidate for a supernatural event. First, no physical cause of paranormal events can be identified; nor, so far as we can tell, are the causes of such events supervenient upon any known physical force or entity. Second, no spatiotemporal cause of such events can be identified. Third, such events defy all attempted scientific explanations. Fourth, paranormal events appear to violate well-established scientific laws. And fifth, paranormal events are highly improbable assuming only known natural causal factors are operating. It is not clear that all forms of paranormal phenomena exhibit apparently purposive or intelligent behavior, however. Michael Scriven offers the following objection to the identification of the paranormal with the supernatural:
[T]he only circumstances under which one might plausibly be said to have demonstrated the existence of a supernatural phenomenon are those in which one has met the criterion for showing that it is not a natural phenomenon of the types so far understood, and also shown that it is so 'different' from those so far understood that it appears to be a case of 'another order of existence', and that it involves some agency or personality... [N]o differences in parapsychology appear greater than those in physics, and the mere involvement of human personality hardly persuades us that we should abandon materialism or naturalistic explanation (Scriven 1976, p. 185).
Thus while Scriven acknowledges that paranormal phenomena do not fall within our known natural categories, his main complaints are that they are not different enough from our known natural categories to warrant being categorized as supernatural and they do not ostensibly involve nonhuman agencies.
Scriven's first complaint is questionable. The ultimate concern of physics is with 'dumb' or impersonal physical interactions between fundamental physical particles and forces (or between undulated strings--whatever the case may be). But parapsychology introduces the possibility of causal interactions between a subject's intelligent and complex psychological states and impersonal and simple physical systems. Thus paranormal phenomena not only defy our known natural categories but, arguably, do involve another order of existence--at least a kind of causal interaction with which we are totally unfamiliar. Paranormal phenomena are different enough from our known natural categories to at least potentially fall within the domain of the supernatural. Or at least Scriven has not given us much justification for thinking otherwise. As for Scriven's second complaint, there is little justification for insisting that the agencies involved in supernatural causation have to be nonhuman. As Paul Dietl points out: " 'Supernatural' implies that the agent be able to bring about events which are exceptions to physical laws. Nothing else about the agent is at issue, however... But he must be a being who can control the laws of nature" (Dietl 1972, p. 236). If a human being can at least apparently control the laws of nature, then we would have an apparent instance of a human agency with supernatural powers.
Nevertheless, Scriven's first complaint does capture an important insight about our concept of the supernatural. That an event is different--even very different--from events which fall under known natural categories may not be enough to for us to reasonably categorize an inexplicable event as a supernatural one. For example, if objects occasionally flew around a room at random apparently for no reason (a form of psychokinesis), lacking any recognizable pattern we would attribute to intelligent behavior, this would be an instance of the paranormal but not an instance of a likely candidate for a supernatural event. Certain instances of telepathy might also be paranormal but not likely candidates for a supernatural event. While Scriven's insistence that nonhuman agency be involved was too restrictive, he did recognize that some form of intelligent agency would have to appear to be involved before we could reasonably categorize a paranormal event as a potentially supernatural event. While paranormal events do not always qualify as likely candidates for a supernatural event, likely candidates for the supernatural always qualify as paranormal events. This is so because the first five of the six criteria for a likely candidate for a supernatural event are also criteria for categorizing an event as a paranormal event. Thus while an ideal survey of the parapsychological evidence for our purposes would focus solely on what Stoeber calls otherworldly-psi, given the general state of the parapsychological evidence available we will see that this is not necessary.
Relative to the lack of evidence for supernatural causation in the distant past, a stronger case for naturalism is provided by the lack of uncontroversial evidence for likely candidates for a supernatural event within recent history. In part, this is because today we have a variety of means at our disposal for providing compelling evidence that an event has occurred or that a phenomenon is real. Multiple video cameras can be set up from different perspectives at a given location to record any events that transpire there. This has been done by security cameras constantly monitoring and recording events occurring on city streets and on commercial and private property, by television news crews documenting ongoing events, and by individuals who happen to be at the right place at the right time. Given the preponderance of video surveillance, unusual events transpiring at various locations have frequently been caught on film. We can even record activity under cover of darkness with infrared cameras which 'see' beyond the visible electromagnetic spectrum.
Despite such technology, no one has ever provided compelling video footage of any likely candidates for a supernatural event. Moreover, there are other ways to provide compelling evidence for likely supernatural intervention in nature. If someone with little scientific knowledge claims to communicate with supernatural agencies and can provide unknown and surprising information about the natural world, we would have good reason to seriously entertain the possibility that this person is communicating with supernatural agencies: "Let a mystic tell us something about the universe that he and no one else knows, and then let that be confirmed by further [scientific] developments, and we can begin to take mysticism seriously. This has never happened" (Stenger 1990, p. 104). By contrast, those claiming to commune with supernatural agencies have provided only obvious or false descriptions of the natural world--if they have described the natural world at all--giving us strong reason to believe that what they take to be evidence of the supernatural is not.
Finally, there is experimental evidence. There is no reason in principle why supernatural agencies could not intervene in the natural world in a regular way without such intervention being subsumed under some kind of natural laws. Paul Dietl provides the following example of just such a situation:
Let us assume that a local prophet opens, or appears with the help of God to open, the mighty Schuylkill River. Two possibilities arise. The first is that the prophet does not figure causally in the natural explanation but that he notices a cue in the physical situation which indicates natural sufficient conditions... [H]e might not be consciously aware of the cue and so might himself honestly believe in the miracle. This sort of explanation can be ruled out, however, if he is required to do miracles at random. Say he allows non-believers to pick twelve miracles and number them. Which one he will do will be determined by the roll of a pair of unloaded dice, and the hour of the day at which it will occur will be determined by a second roll (Dietl 1972, p. 240).
Dietl's example shows that an event that presumably has a supernatural cause does not have to be an extraordinarily rare event, even if it does have to be extremely improbable on the assumption that only known natural causes are present. Despite arguments to the contrary by some philosophers, an event with a supernatural cause does not have to be a unique event. In fact, the example above illustrates that supernatural events could be repeatable on demand. Repeatability is not a feature exclusively reserved for naturally-caused phenomena. As Dietl later points out, if an apparently supernatural event could be replicated in this way, there is no independent variable which could form part of a scientific explanation for such events (Dietl 1972, p. 241). If a prophet succeeded in passing the test Dietl proposes, the only variable correlated with the occurrence of the event would be the prophet's request for it to come to pass. No natural cause could plausibly explain a consistent correlation between the prophet's request and the occurrence of an event.
If during successive out-of-body experiences in a laboratory setting a subject could consistently cause objects to move and bring back information accessible only from a remote location we would have compelling experimental evidence for the existence of a potentially supernatural astral body. If we assume that supernatural causation has occurred within recent history, given such a wide variety of ways of documenting phenomena it is remarkable that there has not been a single uncontroversial case for the existence of any likely candidates for a supernatural event. But if we assume that supernatural causation does not occur, the lack of uncontroversial evidence for likely candidates for a supernatural event within recent history is exactly what we would expect to obtain.
Thus the lack of evidence for likely candidates for a supernatural event in the modern period provides strong inductive grounds for accepting naturalism. What is particularly poignant about this lack of evidence is that several generations of parapsychologists have made systematic attempts to provide uncontroversial evidence of paranormal or 'psi' phenomena yet have failed to do so: "[T]he total accumulation of 130 years' worth of psychical investigation has not produced any consistent evidence for paranormality that can withstand acceptable scientific scrutiny" (Hyman 1985, p. 7). This is why the British parapsychologist John Beloff describes independent corroboration as "the exception rather than the rule" in parapsychological research (Hyman 1985, p. 89). Three features of parapsychological research demonstrate its failure to produce uncontroversial evidence for the paranormal. First, the mainstream scientific community does not recognize parapsychology as a legitimate science. This is not because parapsychologists do not utilize modern scientific methods, but rather because in doing so they have failed to produce unequivocal evidence for the existence of the paranormal. The existence of any paranormal phenomena has yet to be demonstrated to the satisfaction of the scientific community. Parapsychologists may be engaged in a scientific study of nonexistent phenomena.
Second, there is no consensus among parapsychologists about which of the various paranormal phenomena which constitute their subject matter are real:
Which experiments do the parapsychologists regard as producing the best evidence for psi phenomena? According to several critics, the parapsychologists will not, or cannot, answer this question unequivocally. Experiments listed as conclusive by one parapsychologist may be seriously questioned or completely ignored in the listing of another parapsychologist. This is more serious when one realizes that there are only a very few experiments listed by any parapsychologist as conclusive (Ransom 1976, p. 416).
This lack of consensus among parapsychologists, which is even more striking than that found in 'softer' sciences like sociology, is symptomatic of the lack of uncontroversial evidence for the paranormal. In the absence of consistent evidence, parapsychologists simply have no grounds for affirming the reality of any particular paranormal phenomena.
Finally, parapsychology has failed to make any progress in determining the nature of paranormal phenomena. For example, there is no consensus among parapsychologists about what specific properties various paranormal phenomena exhibit. After nearly a century and a half of investigation, parapsychologists are no closer to understanding the nature of the paranormal than they were when psychical research began. As Ray Hyman points out,
[E]ach generation's best cases for psi are cast aside by subsequent generations of parapsychologists and are replaced with newer, more up-to-date best cases. Not only does the evidence for psi lack replicability, but, unlike the evidence from other sciences, it is noncumulative. It is as if each new generation wipes the slate clean and begins all over again. Consequently the evidential base for psi is always shifting. Earlier cases are dropped and replaced with newer and seemingly more promising lines of research (Hyman 1985, p. 86).
This lack of progress in parapsychology is also symptomatic of the lack of evidence for the paranormal. Given no consistent evidence for any paranormal phenomena, parapsychologists can only speculate about what features these potentially nonexistent phenomena might have. Since any events which would qualify as likely candidates for a supernatural event within recent history would fall within the domain of the paranormal (even if some paranormal phenomena should not be regarded as supernatural), the lack of unequivocal evidence for any paranormal phenomena demonstrates that there is no uncontroversial evidence for likely candidates for a supernatural event within recent history.
Moreover, it is particularly interesting that since parapsychology began as 'psychical research' in the 19th century there has been a decreasing interest over time in investigating paranormal phenomena where disembodied agencies allegedly play a causal role. The scientific investigation of paranormal phenomena where nonhuman agencies are alleged to be involved is virtually nonexistent. And while paranormal phenomena allegedly involving disembodied human agents constitute what parapsychologists call survival research, this area of research is minuscule within the field as a whole, with most parapsychologists much more interested in trying to provide unequivocal evidence for extrasensory perceptual modes and psychokinetic abilities. The parapsychologists' overall lack of interest in the role of disembodied agencies in the production of paranormal phenomena may be indicative of the their negative assessment of the prospects for finding any uncontroversial evidence for such instances. If so, the prospects for finding unequivocal evidence for likely candidates for a supernatural event are not promising even by parapsychologists' standards.
In the first part of this essay I considered a variety of ways of defining naturalism and what it means for something to be natural. Naturalism is typically defined as the position that everything that exists is natural. However, a weaker definition of naturalism which retains the fundamental core of the position--that nature is a closed system of natural causes and their effects--is more desirable. Naturalism in the weaker sense maintains that every caused event within nature has a natural cause. In other words, naturalism at least entails that nonnatural causes of events within the natural world--that is, supernatural causes--do not exist. This weaker definition is more desirable because it leaves open the possibility that nonnatural realms (such as a Platonic realm of abstract objects) exist. Thus while naturalists deny the existence of genuine instances of supernatural causation, supernaturalists affirm the existence of such instances.
This understanding of naturalism, however, does not specify what it means for a cause to be natural. I have also considered different theoretical criteria for anything falling within the category 'natural'. My analysis of the concept of the natural has led me to conclude that being physical or supervenient upon the physical and obeying natural laws are necessary and sufficient theoretical conditions for being natural. Being strictly physical, spatiotemporal, and amenable to scientific investigation in principle are sufficient but not necessary theoretical conditions for being natural. From this analysis I have determined that being nonphysical and not supervenient upon anything physical, being nonspatiotemporal, being scientifically inexplicable in principle, and failing to behave in accordance with natural laws are jointly necessary and sufficient conditions for the nonnatural. In theory, any event within the natural world with a cause meeting these four conditions (a nonnatural cause) is a supernatural event.
As they stand, however, these theoretical conditions for the nonnatural are inadequate as practical criteria for marking the natural-supernatural distinction. They do not tell us how to identify a supernatural event in practice. But we can draw a practical distinction by working with criteria which incorporate features common to 'clear-cut' cases of supernatural events with approximations to the theoretical criteria for the nonnatural. This strategy provides us with the following necessary and sufficient conditions for a likely candidate for a supernatural event: (1) the cause of the event cannot be identified as any known physical force or entity nor is it supervenient upon any known physical force or entity; (2) the cause of the event cannot be located in space and time; (3) the event defies all attempted scientific explanations thus far; (4) the event appears to violate well-established scientific laws (as distinguished from genuine laws of nature); (5) the event is highly improbable if it solely has known natural causes; and (6) the event exhibits apparently purposive or intelligent behavior. Being a likely candidate for a supernatural event is a necessary but not a sufficient condition for being a genuine supernatural event. Thus while every likely candidate for a supernatural event may not be a genuine supernatural event, every genuine supernatural event will be a likely candidate for a supernatural event. In other words, a likely candidate for a supernatural event is a potential instance of supernatural causation.
In the second part of this essay I presented a persuasive case for accepting naturalism based on the lack of compelling evidence for any potential instances of supernatural causation. Proponents of the existence of supernatural causes have postulated the intervention of supernatural agencies at various times in the distant past--to explain the origin of the universe, the origin of the Earth, the origin of life on Earth, and the origin of human beings, among other things. But our modern scientific picture of the world grounded in evidence derived from a wide variety of scientific disciplines implies that we live in a naturalistic universe where the only influences on the natural world are impersonal natural causes blindly operating in accordance with fundamental natural laws. As Ernest Nagel wrote:
It is possible, I think, to conceive without logical inconsistency a world in which disembodied forces are dynamic agents, or in which whatever happens is a manifestation of an unfolding logical pattern. In such possible worlds it would be an error to be a naturalist (Nagel 1960, p. 491).
But we do not appear to live in such a world. If we take the broad outline of the history of the universe developed by scientists seriously, we find that the history of life on Earth is a pointless cycle of speciation and extinction driven by savage competition for limited resources. Natural selection removes any role for purpose to play in the origin and diversification of life on Earth. Under the right circumstances, once crude self-replicating molecules emerge the diversification of life follows as blind evolutionary forces take effect. And the prevalent role of extinction and haphazard accidents in the diversification of life on Earth supports the idea that evolution is not a process guided by an intelligent agent working toward an end.
The evidence collected from various sciences has allowed scientists to construct a broad outline of the history of the universe and there is no hint of likely supernatural causation anywhere in this account. Moreover, there is insufficient evidence for likely candidates for a supernatural event within recent history. Our modern scientific picture of the world supports naturalism, but this evidential support was not an inevitable consequence of the nature of scientific inquiry. Scientists could have discovered unequivocal evidence that events for which no plausible natural explanation is forthcoming have occurred either in the distant past or within recent history. But of course they have not made any such discoveries. It is remarkably significant that not a single scientific discovery has falsified naturalism in the sense of making supernaturalism more likely to be true than not when we can imagine several discoveries that would do so. While there are and will certainly continue to be uncontroversial anomalies in our scientific account of the world, none of them have anything to do with supernatural agencies.
 This distinction is still very ambiguous--it isn't clear, for example, that we don't infer the existence of all natural objects to explain objects that we experience. And even if we do not do this, it isn't clear that what we directly experience in perceiving a natural object is the object itself--it may be that all we directly experience in the sight of an object, for example, are reflected photons. Let us put these questions aside and assume that this distinction is sound.
 Many theistic philosophers would deny this, claiming that we can experience God or other spiritual beings directly through mystical experience. Even naturalistic philosophers might concede that we can have direct knowledge of nonnatural abstract objects.
 This should not be taken to imply that these two latter labels refer to the same position. While property dualism is certainly a form of nonreductive physicalism, nonreductive physicalism does not necessarily entail property dualism. Some versions of nonreductive physicalism deny a dualism of properties, maintaining that all properties are at bottom physical properties, while allowing a dualism of descriptions--mental descriptions being irreducible to physical ones. In my discussion of nonreductive physicalism, however, I use nonreductive physicalism in a sense that is equivalent to property dualism--notwithstanding my distinction in terms of states rather than properties.
 See, for example, Paul Teller's "A Poor Man's Guide to Supervenience and Determination" in The Southern Journal of Philosophy Spindell conference supplement 22, pp. 137-162.
 Many contemporary Christians theologians accept some form of physicalism and would probably accept this definition of 'natural'. Nevertheless, most Christian theologians would certainly reject various versions of naturalism using this definition. For example, Christians that affirm the existence of a God who is neither physical nor supervenient upon anything physical would reject any form of naturalism which maintains that everything is physical or supervenient upon the physical. And Christians who affirm the reality of supernatural causation would reject any form of naturalism which maintained that the natural world is solely influenced by physical causes.
 Although I have constrained my discussion of abstract objects to Platonic realism versus nominalism, some philosophers have developed versions of realism about abstract objects which are neither Platonic nor nominalistic, though such positions may ultimately turn out to be incoherent.
 Edwards mistakenly thinks that naturalism entails causal determinism. Since modern physics assumes causal indeterminism on the subatomic level, it would have been better for Edwards to have said that naturalism entails that all caused natural events have natural causes.
 Interestingly, anything within the event horizon of a black hole (where light cannot escape) would also inaccessible in this sense to anyone outside the horizon.
 Suppose that there are also other universes which are not completely causally-isolated from ours but so minimally connected to ours, perhaps through a Planck scale wormhole, that very little causal interaction takes place between the two--perhaps a single exchange of subatomic particles. Arguably, we would be justified in classifying these as two distinct universes, with their very different spatiotemporal or other dimensional frameworks, rather than as different regions of a single universe. If these 'connected' universes would be regarded as natural--we certainly don't want to construe the imagined exchange of subatomic particles as an event with a supernatural cause--then presumably causally isolated universes should be regarded as natural as well, despite the fact that their contents would not be amenable in principle to scientific investigation (to us).
 Presumably, if there are other universes which causally connect to one another such connections would be governed by meta-level laws of nature which may be inaccessible to us.
 I grant that there could be testimonial evidence which would qualify as uncontroversial evidence, such as independently corroborated testimony that has been thoroughly documented.
 For example, the simplest explanation for the lack of evidence for radio signals from technological extraterrestrial civilizations may be that there is no other intelligent life in the universe (this is difficult to say, however--perhaps the radio signals simply have not arrived yet or such civilizations are not intentionally broadcasting signals into space that would be detectable by us). Given our scientific understanding of the origin and diversification of life on Earth, however, the existence of intelligent life elsewhere is not implausible in light of current science. In this case a suspension of judgment would be more reasonable than a rejection of the existence of intelligent life on planets outside our solar system.
 While many parapsychologists believe that paranormal abilities like telepathy and psychokinesis are supervenient upon an organism's brain, given the lack of consistent evidence this hypothesis has merely been proposed, not established by parapsychological research. Moreover, if disembodied spirits can act on the natural world, their influence would appear to us to fall within the realm of psychokinesis; and if they can communicate with us directly, mind-to-mind, any information they provided would have to be provided to us telepathically. If we had some evidence for otherworldly-psi that could not plausibly be reduced to this-worldly forms of the paranormal we would have some evidence for paranormal abilities which are not supervenient upon any known physical system.
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