A Case for Physicalism about the Human Mind (2007)
1. What Is Physicalism about the Human Mind?
2. Physicalism about the Human Mind and Theism
3. Some Evidence for Physicalism about the Human Mind
3.1 An Enumerative Induction
3.2 Evidence from the Neural Dependence of Mental Phenomena
4. An Antiphysicalist Response
In this chapter, I describe evidence for the view that the human mind is a physical entity, in much the same way in which the human digestive system or the human immune system are physical entities. The first section characterizes this view more fully. The second section explains the evidential relevance of physicalism about the mind to theism. The third section sketches two kinds of evidence that support physicalism about the human mind, while the final section considers an antiphysicalist response to the reasoning of the previous section.
1. What Is Physicalism about the Human Mind?
Humans have minds, exemplify mental properties, and undergo mental processes. Some examples of mental properties are thinking that one's keys are in the ignition, hoping that the gas gauge is broken, wishing that the weather will be fine tomorrow, fearing that the repair will be costly, and doubting that all politicians are crooks; other examples of mental properties are smelling the distinctive smell of gasoline (in the absence of actual gasoline), seeing the distinctive blueness of a cloudless sky (in the absence of any blue sky), having an ache in the shoulder, and feeling stressed. Examples of mental processes are figuring out what is causing one's child's fever, planning a trip to Colorado, and deciding to have no dessert.
There's a narrow sense of the word "physical" in which minds, mental properties, and mental processes are clearly not physical phenomena: terms like "mind," "thinking," and "feeling" don't appear in the theories of fundamental physics. In this same narrow sense of "physical," however, a kidney isn't a physical object, respiring isn't a physical property, and digestion isn't a physical process; for the terms "kidney," "is respiring," and "digestion" also don't appear in the theories of fundamental physics. So to concede that mental phenomena aren't physical in this narrow sense isn't to concede very much. Yet surely there's a broad sense of the word "physical" in which kidneys, respiring, and digestion are indeed physical phenomena. What physicalism about the human mind claims is that human minds, mental properties, and mental processes are physical in this broad sense of "physical."
How best to articulate this broad sense of "physical" is controversial in the philosophy of mind. But the view I happen to favor says roughly this:
An individual item (e.g., object, property-instance, or process) is physical in the broad sense if, and only if, it meets either of two conditions: (1) it's an item of a kind that can in principle be defined in the distinctive vocabulary of fundamental physics; or (2) it's a physically realized item of a functional kind.
Condition (1) is straightforward, but condition (2) obviously requires explanation. A functional kind of thing, as I shall understand it, is a kind of thing whose existence consists in the existence of something or other that meets a certain specification, where the composition and working of the "something or other"--the realizer--don't matter, so long as it meets the specification in question. For example, a kidney is a functional kind of object in my sense: for a kidney to exist is just for something or other to exist that can filter blood, no matter what it's made of or how it works. Likewise, respiring is a functional kind of property: for an organism to be respiring is just for it to have some or other property that produces an exchange of oxygen and carbon dioxide between the organism and its environment. And digestion is a functional kind of process: for digestion to occur is just for some or other process to occur--it doesn't matter how--that converts food into useable nutrients. An item of a functional kind is physically realized when, and only when, (i) its realizer can in principle be defined in the distinctive vocabulary of fundamental physics, and (ii) its realizer meets the specification in question solely in virtue of the holding of physical laws and perhaps of other physical conditions. So human kidneys, for example, are very plausibly regarded as physically realized--as items of a functional kind that are (ultimately) realized by vast and unimaginably complex systems of microphysical particles that do the kidney thing by operating in strict accordance with physical laws.
I can now say more precisely how I'll be understanding physicalism about the human mind:
Physicalism about the human mind: Every human mind, every instance of a human mental property, and every human mental process is either (1) an item of a kind that can in principle be defined in the distinctive vocabulary of fundamental physics or (2) a physically realized item of some functional kind.
Thus understood, physicalism about the mind is, of course, an exceedingly abstract view. To make it seem less so, let me sketch an account of what minds, mental properties, and mental processes might be that is consistent with physicalism about the mind. This account strikes me as promising, but I'm not saying it's true, and the truth of this account in particular certainly isn't required by physicalism about the mind; its purpose is just to make it easier to imagine the truth of physicalism about the mind.
Humans have minds, but it doesn't follow that minds are genuine persisting objects. Just as humans have minds, they also have personalities, but no one imagines that human personalities are persisting objects, not even nonphysical ones. In fact, however, physicalism about the human mind can easily treat minds as persisting objects--by treating an organism's mind as its mental system, on a par with its digestive system or immune system. As a rough-and-ready first approximation, we might suggest that an organism has a mental system just in case it houses some complex object or other that can use sensors to form representations about the organism's environment and internal states, that can store some of these representations, and that can undergo internal processes in which these representations interact with one another and with representations of goal-states so as to produce organismic behavior, which behavior, often enough, achieves the organism's goals. To be more specific about the human mind, in particular, we might suggest that an organism has a human-type mental system just in case it houses some complex object or other that meets the conditions already mentioned, but in addition generates representations of states of affairs far removed from the organism in time, space, or magnitude, representations of regularities in the world, and representations of its own representations.
In keeping with this representational conception of the mind, such mental properties as thinking, hoping, wishing, and doubting can be understood as different kinds of mental representing. For example, thinking that one's keys are in the ignition might be a mental property that a human possesses just in case he or she internally represents that his or her keys are in the ignition, where this internal representation plays one kind of causal role with regard to effecting behavior (the belief role) in his or her overall mental system. By contrast, hoping that the gas gauge is broken might be a mental property that a human possesses just in case he or she internally represents that the gas gauge is broken, where this representation plays a different kind of causal role with regard to effecting behavior (the hope role) in his or her overall mental system. And a very similar treatment can be given for such mental properties as smelling the distinctive smell of gasoline (in the absence of actual gasoline) and having an ache in the shoulder. Smelling the distinctive smell of gasoline (in the absence of actual gasoline) might be a mental property that a human possesses just in case he or she internally represents (in this case, misrepresents) the presence of gasoline vapor, but does so by using a system of mental representation importantly different from the system used when thinking that gasoline vapor is present. Having an ache in the shoulder might be a mental property that a human possesses just in case he or she internally represents damage in a certain subregion of his or her shoulder region, but does so by using a system of mental representation importantly different from the one used when thinking that a part of his or her shoulder region is damaged. The different intentional contents of different mental representations (i.e., the fact that thinking that one's keys are in the ignition isn't the same as thinking that it'll rain tomorrow) can be accounted for by appeal to the different causal or nomological or historical relations that the different mental representations bear to the different states of affairs external to the organism.
Mental processes such as figuring out what is causing something or deciding to act can also be understood in keeping with the representational conception of the mind. Such processes can be identified with particular kinds of causal sequence involving mental representations. For example, figuring out the cause of a child's fever might involve mentally representing the possible causes of the fever, deriving mental representations of the observable phenomena that each possible cause would produce, and then searching for stored mental representations of actually observed phenomena that rule out every causal hypothesis bar one. Such a mental process, despite its occurring in accordance with causal laws, could still be a rational or reasonable one in virtue of its reliability in generating true answers to causal questions. Similarly, deciding to act might involve mentally representing a goal-state, mentally representing various action-outcome relations that hold in the world, using a trial-and-error procedure to generate a mental representation of a sequence of actions that will eventually result in the represented goal-state, and then storing a mental representation of the first action in that sequence in a buffer whose contents feed into a subsystem that controls bodily movements. Such a process, despite its occurring in accordance with causal laws, could still amount to a free decision in virtue of its meeting further conditions, e.g., its occurring through the operation of properly functioning mechanisms of deliberation, or the mentally represented goal's being a goal that one approves of oneself having.
2. Physicalism about the Human Mind and Theism
Physicalism about the human mind is logically consistent with theism. That's because, if mental phenomena are functional phenomena in the liberal sense of "functional" sketched in section one, then they might be physically realized in the human case but nonphysically realized in the divine case. However, physicalism about the human mind is still evidentially relevant to theism. The hypothesis of theism, as typically understood by philosophers, says there exists a special, nonhuman kind of person--God--who enjoys unlimited versions of the capacities to know, to love, and to decide to act that human persons possess. But since these capacities are mental, and since God is supposed to have no physical body or brain, the hypothesis of theism must say there exists a nonphysical and nonphysically realized mind. Now, if we want to assess the probability of theism given the available evidence, we must first assess its prior probability, i.e., its probability given our background knowledge. However, given theism's commitment to a nonphysical and nonphysically realized mind, its prior probability will be raised if we already know of examples of nonphysical and nonphysically realized minds (even if those minds, unlike the hypothesized divine mind, are finite). Thus, if we learn that human minds are neither physical nor physically realized, the prior probability of theism will get a boost. By the same token, however, if we learn that human minds are physically realized, so that we don't already know of any examples of nonphysical and nonphysically realized minds, then the prior probability of theism will fail to get that boost. To the extent that physicalism about the human mind rather than dualism is well evidenced, therefore, theism should be assigned a lower prior probability.
Is physicalism about the human mind evidentially relevant to theism in any other way? I think it is. To avoid being disconfirmed by the existence of moral evils (and perhaps for other reasons as well), theism requires that human agents freely choose to act in one way rather than another. But so-called compatibilist accounts of free choice--which say that what makes a choice free is not its being causally undetermined but rather its being causally determined via a special kind of causal route--will not suit theism's needs. For every compatibilist account of free choice, to be plausible, must insist that no human choice is free if the causal chain culminating in the choice can be traced back to an intentional agent who intended or at least foresaw that choice. But if God created the world and instituted the causal laws that govern its workings, then all human choices result from causal chains that can be traced back to an intentional agent who intended or at least foresaw those choices, and so no human choices are free. What theism requires, therefore, is that human agents freely choose in some way that entirely removes the free choice from the nexus of laws of nature, be those laws deterministic or statistical. However, if physicalism about the mind is true, then human choices are as much a part of the nexus of laws of nature as any other events in the physical world. So if physicalism about the mind is true, then theism cannot have what in fact it requires.
3. Some Evidence for Physicalism about the Human Mind
Whether physicalism about the human mind is true turns on whether there exist any human minds that aren't physically realized; and the existence of such contingent entities as nonphysically realized human minds is a poor candidate to be known a priori. So, it's hard to see how there even could be an a priori case for physicalism about the human mind (or against it, for that matter). In this section, therefore, the case I sketch for physicalism about the human mind is an a posteriori one. Moreover, although an a posteriori case could in principle take the form of a deduction from a posteriori premises, this case for physicalism about the human mind is irreducibly inductive: it consists of mustering two bodies of empirical fact each of which raises the probability of physicalism about the human mind (but without raising that of dualism too).
3.1 An Enumerative Induction
There is a strong empirical case for holding that concrete phenomena of many different kinds--chemical, biochemical, cell-biological, histological, physiological, geological, meteorological, and astronomical--are physical or physically realized. Indeed, there is a strong empirical case for holding that all concrete phenomena that aren't mental phenomena or--like social-scientific phenomena--partially constituted by mental phenomena are physical or physically realized (see Melnyk 2003a, ch. 6.1-6.6).
Of course, that all nonmental phenomena are physical or physically realized doesn't deductively entail that mental phenomena are too. But it can still be some evidence that they are; and it is. Consider an analogous case: the discovery that all noncarnivorous plants are biochemically realized is clearly some evidence that all plants, including therefore all carnivorous ones, are biochemically realized. (Each biochemically realized species of noncarnivorous plant is a so called positive instance of the universal hypothesis that all plants are biochemically realized.) Likewise, the discovery that all nonmental phenomena are physical or physically realized is some evidence that all phenomena, including therefore all mental phenomena, are physical or physically realized. (Each physical or physically realized kind of phenomenon is a positive instance of the universal hypothesis that all phenomena are physical or physically realized.)
Some might object that nonmental phenomena are the sorts of thing you'd be quite unsurprised to discover are physical or physically realized, whereas just the opposite is true of mental phenomena; so evidence about nonmental phenomena isn't relevant to the case at hand, i.e., mental phenomena. In truth, however, the fact that nonmental phenomena have turned out to be physical or physically realized is very surprising. Superficially, the nonmental world exhibits a breathtaking diversity--just think, for example, of the thousands of species of plants and animals or the dozens of kinds of rock that are discernible by casual observation alone, not to mention the vast numbers that systematic inquiry has revealed. Yet if all nonmental phenomena are physical or physically realized, then beneath this incredible diversity lies a profound unity; underlying them all are fundamental physical phenomena. But the world needn't have been that way. Each different kind of rock, for example, might have turned out to be unique, sharing no constituents with any other kind of rock, still less with living things; each animal species might have turned out to be made of its own unique kind of matter; and so forth, as far as imagination permits. Moreover, not only was this profound unity in nonmental phenomena but one possibility among many, it was not in fact expected. For example, Aristotle, and the long tradition of physical science he inspired, didn't even think that celestial and terrestrial phenomena shared basic constituents! The scientifically trained philosopher C.D. Broad, in 1925, could claim chemical phenomena to be plausible instances of phenomena that are emergent relative to physical phenomena, and hence not physically realized. The idea that such life processes as metabolism and growth are biochemically and hence physically realized was resisted well into the twentieth century. Any feeling that nonmental phenomena are the sorts of thing you'd be unsurprised to discover are physical or physically realized merely reflects the penetration into ordinary thinking of centuries of hard-won scientific discoveries; in hindsight, many truths (not all, of course) seem obvious.
3.2 Evidence from the Neural Dependence of Mental Phenomena
As far as we can tell, all human mental phenomena are dependent on neural phenomena; we never catch the human mind at work without also catching the human brain at work. Since this dependence is best explained by physicalism about the human mind, it's evidence that physicalism about the human mind is true. In a nutshell, that's the reasoning of the present subsection.
More carefully expressed, the finding to be explained is this: for any (human) person you like, and for any mental state or mental process that person might be in or might undergo, in order for that person to be in that mental state or to undergo that mental process, there is neural activity of some distinctive kind that has to be going on--simultaneously--in that person's brain. In the numerous studies that support this finding, a brain-imaging technique is applied to the brain of a human experimental subject who has been subjected to some stimulus (e.g., spoken words, objects placed in the hand) or instructed to perform some mental task (e.g., to read silently, to clench a fist). What is found in the studies is that in each subject distinctive regions of the subject's brain are especially active when a particular stimulus is presented or mental task performed. (Often, of course, the distinctive region is the same in all subjects for each stimulus or task; but not invariably.) Of particular importance is the very wide range of mental tasks that experimental subjects in such studies are asked to perform. They include mentally rehearsing a learned motor task, doing mental arithmetic, attending to an unstimulated body part, and visualizing a scene. Although these mental activities seem far removed from the processing of sensory input or the generation of motor output, they too don't occur without accompanying neural activity of a distinctive sort.
This remarkable dependence of the mental on the neural is simply explained by physicalism about the human mind, since it's an almost immediate consequence of physicalism about the human mind. For if all human mental phenomena are physically realized, then presumably they're realized by human neural phenomena (perhaps plus physical environmental conditions); and if all human mental phenomena are realized by human neural phenomena, even in part, then whenever someone exhibits a mental phenomenon, he or she must exhibit some neural phenomenon too--the neural phenomenon that (in part) realizes the mental phenomenon. Moreover, the explanation of the neural dependence of the mental that physicalism about the human mind can provide is the best available explanation, since, for two reasons to be given shortly, it's a better explanation than its dualist rival. Dualism about the human mind, in taking human mental phenomena to be numerically distinct from and not even realized by neural phenomena, has to explain the neural dependence of human mental phenomena by hypothesizing that human mental phenomena are connected to human neural phenomena by hitherto unrecognized--and physically irreducible--laws of nature. The reason why we don't ever catch the human mind at work without also catching the human brain at work is that these laws of nature yoke every human mental phenomenon to some or other human brain phenomenon.
This dualist explanation is inferior to the physicalist explanation because, first, it's less economical in its ontological commitments, and ontological economy is a good-making feature of an explanation. The dualist explanation, precisely because it's dualist, treats mental phenomena as existing over and above their associated neural phenomena: if dualism is true, then a divine creator who'd already created the associated neural phenomena would have to do extra creative work to produce mental phenomena. The physicalist explanation, by contrast, treats mental phenomena as perfectly real, but as physical phenomena or as functional phenomena that are realized by their associated neural (hence physical) phenomena: if physicalism about the human mind is true, then a divine creator who'd already created the neural phenomena associated with human mental phenomena (in appropriate physical environmental conditions) would have to do no more work to produce mental phenomena. If human mental phenomena are one and the same as certain neural phenomena, then the reason why the creator would have to do no more work to produce mental phenomena is that if X is one and the same as Y, then to create X is to create Y. If mental phenomena are instead realized by certain neural phenomena (plus physical environmental conditions), then the reason why the creator would have to do no more work is that if they are so realized, then, given the laws of physics, those neural phenomena (plus appropriate physical environmental conditions) must meet functional job descriptions the meeting of which by some things or other just is the existence of the realized mental phenomena. Hence, if the realizing neural phenomena (in appropriate physical environmental conditions) exist, and if the laws of physics hold, then the realized mental phenomena must exist too (in the strongest sense of "must").
The second reason why the dualist explanation of the neural dependence of the mental is inferior to its physicalist rival is that the dualist explanation is not only committed to the laws of physics (to which the physicalist explanation is obviously committed as well) but also to a huge number of physically irreducible laws of nature connecting human mental phenomena to the human neural phenomena on which they depend. By contrast, the only fundamental laws to which the physicalist explanation is committed are the laws of physics. The dualist explanation's commitment to these additional laws--laws that are irreducible to the physical laws that its rival acknowledges--is, of course, a second reason why the dualist explanation is less economical than its physicalist rival, and hence a less good explanation. In addition, however, these irreducible mental-to-neural laws prompt puzzling questions to which no answers are forthcoming. Why are nonphysically realized human mental phenomena connected by law to the particular human neural phenomena to which they are (allegedly) in fact connected, rather than to others? Why do all nonphysically realized human mental phenomena have to be connected by law to particular physical phenomena at all? Why can't nonphysically realized human mental phenomena mostly interact among themselves, with just some of them sensitive to the outputs of the neurons running to our brains from our sensory transducers and others of them supplying the inputs to the neurons that run from our brains to our muscles? In short, why do nonphysically realized human mental phenomena need a brain at all? These questions don't arise for the physicalist explanation of the neural dependence of the mental, since it's not committed to the laws that prompt them.
4. An Antiphysicalist Response
One possible response to the preceding section does not deny that the evidence presented there favors physicalism about the human mind over dualism. Instead, it claims that dualism enjoys some advantage in credibility over its physicalist rival, some advantage great enough to outweigh the advantages which physicalism was shown to enjoy over dualism in the preceding section. But what might that advantage in credibility be? In my view, there turn out to be no promising candidates.
Could dualism about the human mind enjoy an advantage over its physicalist rival in explanatory power? That is, could there be some aspect of human behavior for the causal explanation of which it was necessary to suppose that its mental causes weren't physically realized? In principle, I say, there could be such an aspect of human behavior; but in practice, there very probably isn't. There could be in principle, because if human physiologists had known that human behavior is caused by signals carried by motor neurons that run from the skull, and had then opened up some human skulls for the very first time only to find that they were quite empty, or merely full of blood, then the positing of a nonphysically realized mind to explain the complexities of human behavior would surely have been unavoidable; there would simply have been no physical candidate of sufficient complexity to do the explanatory job.
The reason why there very probably isn't in fact any aspect of human behavior for the explanation of which it's necessary to posit dualistically construed mental states takes a little longer to explain. The proximate cause of human behavior is the contraction of our muscles, which contraction is caused in turn by the firing of motor neurons, which firing is caused in its turn by the firing of neurons in the brain, and so forth. But there's abundant evidence for thinking that the various kinds of neurons, as well as the specialized cells that make up our muscles, are physically realized (i.e., realized by fundamental physical phenomena governed by fundamental physical laws); so they aren't at all special biochemically or fundamental-physically. There's also abundant evidence for thinking that all fundamental physical phenomena, to the extent that they can be explained at all, can be explained by appeal only to fundamental physical phenomena and laws. (The evidence is simply the track record of successful fundamental physical explanation of a wide variety of fundamental physical phenomena.) So it looks like there's no human behavior for the causal explanation of which it's required to assume the existence of anything that's neither physical nor physically realized. A fortiori, it looks like there's no human behavior for the causal explanation of which it's required to assume the existence of anything mental that's neither physical nor physically realized. To put it in a nutshell, the dualistic interpretation of ordinary mental phenomena as neither physical nor physically realized is a hypothesis which, from the standpoint of causal explanation of nontendentious phenomena, we can do without. And so the prospects are dim for dualism about the human mind to gain any advantage in explanatory power over its physicalist rival.
It strongly seems, therefore, that if dualism about the human mind is to enjoy any advantage in credibility over its physicalist rival, the advantage will have to derive from some a priori source. And, indeed, the most discussed argument for dualism in recent years has been a priori in character: David Chalmers' zombie argument (1996). I find this argument inconclusive, but to explain why would take another chapter.
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 Most of the material in this chapter is presented more fully, carefully, and rigorously elsewhere (see Melnyk 2003a, chs. 5 & 6; 2003b). Here, I sacrifice some detail, care, and rigor for concision and, I hope, accessibility.
 What I say in this and the following paragraph is a much simplified version of ch. 1 of Melnyk 2003a.
 The representational theory of mind sketched here is developed and defended by Jerry Fodor; see, for example, his 1975; 1986.
 The representational theory of phenomenally conscious mental states sketched here is developed and defended by Michael Tye; see, for example, his 1995.
 For different theories of how nonrepresentational and possibly physical facts might determine the representational content of mental states--even when mental states misrepresent--see, for example, Millikan 1984; 1993; Dretske 1988; Fodor 1990.
 For more on reasoning of approximately this kind, see Philip Kitcher's account of eliminative induction in his 1993, ch. 7.4.
 For elaboration of this idea, see, for example, Alvin Goldman's (1986) discussion of reliabilist epistemology in his 1986 and Larry Laudan's discussion of scientific methodology in his 1996.
 There is a huge literature on so-called compatibilist accounts of free decisions, i.e., those accounts according to which what makes a decision free is not its being causally undetermined but rather its being causally determined via one kind of causal route and not another.
 Thanks to Paul Draper for suggesting this line of argument to me.
 For a slightly fuller presentation, see Melnyk 2003a, 283-285. The line of reasoning apparently originated with J.J.C. Smart; see his 1959, 142.
 For a fuller presentation, together with sources for the experimental results reported, see Melnyk 2003a, 298-304. I first encountered this line of reasoning (albeit used for a slightly different purpose) in Churchland 1988, 20 & 28.
 The parenthetical phrases in the preceding sentence are included because most philosophers of mind are so called externalists about mental properties, holding that mental properties are typically such that an organism possesses them only if it's appropriately related to an environment of a certain kind; for example, no organism can think thoughts about water unless in the organism's environment there does exist or has existed actual water.
 For defense of the claim that ontological economy is a good-making feature of an explanatory hypothesis, see Melnyk 2003a, 245-251.
 For evidence to support this claim, see Melnyk 2003a, Ch. 6.5.
 For a full defense of the claims in this paragraph, see Melnyk 2003b.
 See Melnyk 2001. I am grateful to Paul Draper for helpful comments on earlier drafts of this chapter.
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