David Chalmers' Principle of Organizational Invariance and the Personal Soul (2014)
David Chalmers argues that conscious experience is a real but nonphysical feature of nature. However, he also believes that all particular facts about any conscious experience supervene (naturally, but not logically) on physical facts, such that physical facts fully determine any conscious experience. His principle of organizational invariance goes even further to claim that fine-grained functional organization fully determines any conscious experience (naturally, but not logically). This principle has powerful implications for artificial intelligence, allowing for the possibility of fully conscious digital computers. But the principle of organizational invariance is not compatible with the concept of a personal soul. This paper does not attempt to prove or disprove the existence of a personal soul, but defends its conceptual coherence against the challenge presented by Chalmers' principle of organizational invariance.
1. Conscious Experience
Chalmers characterizes conscious experience far more deeply than do strict materialists (those physicalists who hold that all substances and properties are physical). He distinguishes conscious experience from associated phenomena such as cognition and awareness. In the literature his concept of conscious experience is also called phenomenology and sentience. One of his main concerns is the common conflation of psychological features (such as cognition, introspection, awareness, etc.) with phenomenal ones (the experiential "feel" of a conscious mental state) (Chalmers, "Conscious" p. 30). Though closely associated with psychological phenomena, conscious experience is truly a distinct phenomenon in itself, as Chalmers takes great care to show. On his "naturalistic dualism" (Chalmers, "Conscious" p. 128) conscious experience is a fundamental property of nature (Chalmers, "Conscious" pp. 127-128). He contends that conscious experiences are related to physical phenomena through a system of natural, yet-to-be-discovered psychophysical laws—hence experiential features of mind "naturally" rather than logically supervene on physical facts (Chalmers, "Conscious" pp. 127-129).
2. Principle of Organizational Invariance
Although Chalmers views conscious experience as a further fact of nature "over and above" physical phenomena, he also believes that it naturally supervenes on physical phenomena. This means, first, that if two systems are physically identical, then they will have an identical conscious experience. Second, it means that two systems with the same fine-grained functional organization will have an identical conscious experience. We don't have to worry about many details, such as the precise arrangement of atoms or the spin state of every electron, to determine what conditions would produce two systems with an identical conscious experience. All that is necessary is that both have the same fine-grained functional organization (Chalmers, "Conscious" pp. 248-249). On this principle the substrate by which the fine-grained functional organization is realized is irrelevant to a person's conscious experience. In place of a biological, carbon-based brain, precisely the same conscious experience could be realized in silicon chips or a system of beer cans and Ping-Pong balls (Chalmers, "Conscious" p. 249).
3. Strong AI
Chalmers' principle of organizational invariance entails the possibility of strong artificial intelligence (AI). According to strong AI, for any given conscious experience, there exists some computer program such that any implementation of it will fully duplicate that conscious experience. It is important to note here that it is the physical implementation of the program, rather than the program itself, that would give rise to conscious experience. The physical implementation has the causal dynamics to give it semantic content, which a mere sequence of symbols (like a program) would lack (Chalmers, "Conscious" p. 327). On the principle of organizational invariance, conscious experience is independent of how the program is physically implemented, but it must be physically implemented in some way. Chalmers makes an extensive argument for strong AI, and responds to various objections to strong AI, in chapter 9 of his book (Chalmers, "Conscious" pp. 313-332).
4. Rights of Animals and Avatars
In his arguments against strict materialism, Chalmers presents the logical possibility of a phenomenal zombie—a person who is physically and behaviorally identical to a conscious person, but lacks conscious experiences (Chalmers, "Conscious" p. 95). As Chalmers describes it, for such a zombie "all is dark inside" (Chalmers, "Conscious" p. 96). Chalmers thinks that phenomenal zombies are logically possible, but not naturally possible (i.e., they are conceptually coherent, but cannot be realized in the actual world because they are prohibited by psychophysical laws of nature).
When considering animal sentience, the concept of a phenomenal zombie is no mere philosophical musing. Questions about conscious experience have significant moral implications for animal rights. For example, if a certain animal's mind was purely 'psychological' (in Chalmers' sense), it would lack conscious experiences and thus any capacity for real suffering. Signs of extreme discomfort could be purely 'psychological' effects with no conscious experience underlying them.
When a dog howls in pain, we tend to believe that it is the subject of a very unpleasant conscious experience. Hence some laws impose stiff penalties—such as imprisonment—to protect companion animals against cruel treatment. These laws assume that dogs and at least some other animals have conscious experiences. However, in the case of a lobster's desperate struggle in a pot of boiling water, it is common to assume that no conscious experience is involved. No judge charges anyone with a crime for boiling a lobster alive. Yet, perhaps the lobster's desperate struggle may be more than just a mechanical reaction to the way it is being stimulated. Just as many believe a dog's painful howl is an indication of a painful conscious experience "on the inside," could not the lobster's desperate struggle be an indication of its conscious experience? Should people who boil lobsters alive be sent to prison? We have no experience meter to determine which species have conscious experiences and which do not (Chalmers, "Conscious" p. 239). I think we make assumptions based partly on how we emotionally relate to them.
I introduce the possibility of suffering in psychologically simpler organisms to consider what sort of maximally simple system could have a high intensity of conscious experience. If only humans have conscious experiences, then determining whether strong AI is indeed naturally (i.e., nomically) possible would be most difficult. The possibility of conscious experiences in much simpler organisms would simplify the issue. If strong AI is naturally possible, then a computer with the right kind of processing could be the subject of intense conscious experiences. This would present serious ethical issues for the future of computing. Suppose video games become so advanced that they include high intensity conscious experiences. Unscrupulous video gamers may enjoy playing with the high stakes that their avatars may experience intense pleasure or intense torment as a consequence of their successes or failures against their avatar opponents. Perhaps the avatars could be designed to experience profound intensities of pleasure or torment with only a primitive level of intelligence. Animal rights activists might then expand their scope to protect entities in a computer. (People for Ethical Treatment of Animals could expand its name to People for Ethical Treatment of Animals and Avatars.) But if Chalmers' principle of organizational invariance is false and no implementation of a computer program can have conscious experiences, then all concerns about computer suffering would be empty because nothing in a computer could be the subject of true suffering.
5. Chalmers' Principle and the Possibility of a Personal Soul
Chalmers' principle of organizational invariance leaves no room for anything other than fine-grained functional organization to determine any personal property of conscious experience. Consequently, it presents a challenge to the concept of a personal soul. But before elaborating on exactly what it challenges in that concept, I must characterize a personal soul hypothesis below.
6. A Personal Soul Hypothesis
There is a great deal of controversy as to whether people have a personal soul that survives the death of the physical body in some sort of afterlife. The controversy is complicated by the diversity of the definitions of a soul. Setting aside the possibility of its postmortem survival, let us consider the intuitive concept of the soul that the personal soul proponent likely has in mind. Although it often has strong religious connotations, a personal soul hypothesis can be stated in secular terms, making no assumptions as to whether the soul survives physical death. This minimalist hypothesis can be defined as the position that all of a person's conscious experiences have a common property that is unique to the person. I shall refer to this hypothesis as the personal soul hypothesis.
The personal soul hypothesis is a hypothesis about conscious experience that does not define the personal soul itself. For the purposes of this paper, though, I shall define the personal soul as that aspect of conscious experience that is associated with a person in the sense that all of the person's conscious experiences have some common property that is unique to the person. There is one sense in which conscious experience is a general phenomenon applicable to all persons. But if the personal soul hypothesis is true, then there is a second sense in which there is a particular conscious experience specific to each person. The personal soul is constituted by conscious experience in this second sense.
It may be helpful here to distinguish a person's conscious experiences from the person's phenomenon of conscious experience: particular conscious experiences are manifestations of the person's phenomenon of conscious experience. In the general sense, conscious experiences would be any manifestations of the phenomenon of conscious experience. But on the personal soul hypothesis, all manifestations of a person's phenomenon of conscious experience have a common property that is unique to the person. And the reason for this is because the person's phenomenon of conscious experience itself has some intrinsic property that is unique to the person.
7. Multiverse Analogy
The personal soul hypothesis has an interesting analogy in multiverse theory. Multiverse theorists believe that the physical constants of our universe froze out within about a millionth of a second from its beginning (Stenger, "Timeless" pp. 324-325). They believe in a multitude of other universes besides ours whose physical constants froze out differently from ours. Within a universe, two processes may differ greatly in time, space, and function, but the physical constants involved in the implementation of those processes are identical. Likewise, on the personal soul hypothesis a mind may implement a wide variety of cognitive processes, but the great diversity of associated conscious experiences will have some common property that is unique to the person.
8. Reasons for the Personal Soul Hypothesis
At this point I will make no effort to argue for the truth or falsity of the personal soul hypothesis. Instead, I will show how the hypothesis fits the intuitive concept of most personal soul proponents.
8.1 Independent of Awareness Content
I suspect that nearly all of those who believe that babies have souls also believe that, once grown up, they will have the same soul that they had when they were babies. Consider a pair of healthy identical twin sisters, Alpha and Beth. Next, consider each of them at two very different stages of life. Let "Early Alpha" signify Alpha at some moment during her first month of having conscious experiences and "Early Beth" signify Beth at some moment during her first month of having conscious experiences. In addition, let the moment for Early Alpha and Early Beth be chosen such that the content of their awareness is most similar. Next, let "Mature Beth" signify Beth at forty years of age, healthy and intelligent, at a moment when the content of her awareness is most dissimilar to the content of Early Beth's awareness. A person's awareness content changes radically over the years of development from earliest presence of conscious experience to maturity. In terms of awareness content, Early Alpha is much closer to Early Beth than is Mature Beth. But if we were to define a soul in functional terms, then Early Alpha would be a much better candidate for the soul of Early Beth than would Mature Beth—contrary to the personal soul hypothesis. This would be consistent with David W. Shoemaker and Derek Parfit's conclusions about personal identity (Parfit, "Reasons"; Shoemaker, "Incoherence" pp. 199-243), in the sense of there being no coherent or meaningful concept of a soul, as their arguments are based on physical facts alone. This would also be consistent with Chalmers' view, as he believes that all particular facts about a conscious experience naturally supervene on physical facts. However, according to the personal soul hypothesis, Early Beth would have precisely the same soul as Mature Beth and be fundamentally different from Early Alpha.
8.2 Independent of Feelings, Emotions, and Memories
Suppose that a person experiences great suffering in this life but wants to continue to exist in an afterlife. Such a person might well want to forget past suffering and enjoy peace and happiness for eternity. If the soul becomes a different soul due to the change of memories, emotions, and feelings, such a person could have no hope of a desirable afterlife. The only possible survival of such a soul would entail a continuation of the same old suffering. On the personal soul hypothesis, though, there is no need for a person to continue to experience past feelings or emotions to ensure that the person being rewarded after death is the same person that suffered in the past. The person would not even need to retain any memory of past experiences. All that would be required is the continuation of conscious experience with the same personally unique property.
8.3 Independent of Dispositions
Those who believe in an eternal afterlife for souls expect the same soul to continue from this life to the next. They do not believe that their current souls will be replaced by different souls in the afterlife. Otherwise, one would cease to exist with someone else taking his place in the afterlife. And when a person who is accustomed to pursuing perverse pleasures becomes convinced to repent and gains hope of eternal life, he doesn't repent with the expectation that his past soul will go to Hell while a new soul will go to Heaven, or that his previous perverse dispositions will be present in the afterlife. He likely expects the same soul to continue in the afterlife, but with radically transformed dispositions. Just as a universe can support a great diversity of processes under precisely the same physical constants, so too could all of a person's conscious experiences have the same, personally unique property, despite being associated with processes shaped by a wide variety of dispositions within the person's mind.
8.4 Independent of Brain Condition
Suppose that a person is born with a severely defective brain and its accompanying mental deficits. If the soul is defined in terms of its functionality, such a person would need to retain those deficits in Heaven in order to remain the same person. But on the personal soul hypothesis, the person with a severely defective brain in earthly life could have a highly advanced mind in Heaven. Similarly, during earthly life he could have precisely the same soul as before despite attaining great developmental advances due to futuristic brain procedures to correct his mental deficits. And his soul would still remain the same despite progressive brain degeneration leading to advanced Alzheimer's disease.
The following analogy might be helpful. A pure gold doorknob could be melted down and made into a pure gold doorstop. In this transformation, the object ceases to be a doorknob and becomes a doorstop instead. However, it never ceases to be pure gold. The person undergoing radical brain changes might become a radically different person in a functional sense, but the nonphysical soul would remain unchanged by the physical brain transformation.
8.5 Sense of Self
There is a 'psychological' sense of self that is fully explainable without regard to conscious experience (Dennett, "Consciousness" pp. 412-430). However, there is also an intuitive sense of conscious experience as something deeply personal that no one else can experience in the same way. And it is not just a general feature that is shaped by physical conditions alone. That is, there is something intrinsic to a person's conscious experience that determines what it is like to be that person over and above extrinsic factors of that person's physical or mental configuration. The personal soul hypothesis is fully consistent with this intuitive sense of self. According to Chalmers' natural supervenience thesis, conscious experience is a general feature that is fully determined by the physical (or structural) conditions that give rise to it. On this supervenience thesis and his principle of organizational invariance, though, any intuitive sense of conscious experience as deeply personal must be an illusion of the psychological sense of self.
9. Challenges to the Personal Soul Hypothesis
9.1 Divided Minds
Cases of callosal disconnection (where the corpus callosum connecting the two cerebral hemispheres is severed) present the challenge of a possible single center of consciousness being split into two separate centers of consciousness. However, such "split-brain" cases do not necessarily present a serious problem for the personal soul hypothesis as I have defined it. The problem of separate centers of consciousness would challenge the personal soul hypothesis only if there were no personally unique property of conscious experience in the split-brain patient. Even with two separate centers of consciousness in one brain, the personal soul hypothesis would be unscathed so long as the conscious experiences associated with those two centers had the same personally unique property. Callosal disconnection would then be no more problematic than the present self being unaware of the future self, or of the present self forgetting some things about the past self.
A cosmological analogy is also instructive here. Events widely separated in space and time act according to precisely the same physical constants within a universe. Similarly, the personal soul hypothesis could be true even if there were multiple centers of consciousness in one brain that do not share the same personally unique property of conscious experience—for example, if one brain was a habitation for multiple souls (each of which having within it a common property unique to that particular soul). This would be akin to possession by multiple demons.
Up until now I have been assuming that the same soul is not simultaneously associated with separate biological systems. For example, in the identical twin example we've presumed that Alpha and Beth are two different persons with different souls. Although the personal soul hypothesis technically allows two separate biological systems to have the same soul, they would then have to be regarded as the same person. 'Splitting' could become even more complex if a person could be split between multiple biological systems, each of which is a habitation of multiple souls. Our experience of the unity of consciousness makes the splitting of a person between separate systems highly counterintuitive, however. Even conjoined twins who share enough brain tissue to be able hear one another's thoughts or see each other's dreams would be considered two distinct persons by virtue of their distinct unified consciousnesses.
9.2 Physical Boundary Problems
The possibility of transplanting a part of one person's brain to another person's brain could present a significant challenge to the personal soul hypothesis. Would conscious experiences associated with processes in the transplanted portion have the intrinsic personal property of the donor, the recipient, or some newly created person? This question would need to be answered when testing the personal soul hypothesis. As of this writing, there is no technology available to perform such testing.
A great mystery on the personal soul hypothesis is why a particular physical system, whether an organic brain or a system of Ping-Pong balls and beer cans, is in an intimate association with a conscious experience having a personally unique intrinsic property that remains in association with that particular system and not any other. There are temporal boundary issues: when does a person's soul first begin, and when does it finally cease to be in association with physical processes within the person's biological system? There is also a question as to whether the soul is in association with any processes at all during times of unconsciousness. Chalmers' natural supervenience thesis avoids such spatiotemporal boundary problems because it lacks any personally unique phenomenon that would have its associations confined to any particular physical system. However, there is a readily observable phenomenon that presents a parallel boundary enigma. A person enjoying a moment of a rich visual experience is not a vast collection of subjects, each having its own microscopic boundary of conscious experience. Rather, a single subject takes in the whole scene as one big conscious experience. One might argue that the person makes progressive changes from a subject at one moment to a different subject at the next moment, but it would be difficult to defend the view that there are many tiny subjects of visual conscious experience within a short moment. Chalmers' principle of structural coherence—that "the structure of consciousness is mirrored by the structure of awareness, and the structure of awareness is mirrored by the structure of consciousness" (Chalmers, "Conscious" p. 225)—may well lead us to expect that well-coordinated parallel processing would result in a unified conscious experience. But this falls short of explaining why a single subject of conscious experience could be spread over a vast number of brain cells. The structural coherence principle itself has yet to be explained in terms of fundamental laws (Chalmers, "Conscious" pp. 276-277). And the physical boundary problem for the personal soul hypothesis is similar to the boundary problem for rich visual conscious experience, but it is extended to include all of the person's conscious experiences throughout the person's lifetime.
It is important to note here that the concept of a personal soul residing within some physical boundaries does not necessarily imply that the personal soul is some kind of ectoplasm or ghost in the machine. Likewise, multiverse theory does not propose that the physical constants of a universe are a ghost in the universe. Just as multiverse theory proposes that each universe has its unique set of physical constants, the personal soul hypothesis proposes that all of a person's conscious experiences have some common property that is unique to the person. Whereas a universe's physical constants actually do determine the outcome of physical processes, the personal soul hypothesis does not go this far. It would be entirely consistent with the hypothesis if a person's unique property determined nothing about the outcome of any physical processes at all.
9.3 Interactionist Dualism and the Paradox of Phenomenal Judgment
When contemplating mental causation, consider the statement: "We judge that we have conscious experiences because we have conscious experiences." Cause is at the root of the word because. The statement would seem to indicate a cause and effect relationship between our conscious experiences and our judgments that we have conscious experiences. The view that consciousness is a cause that has an effect on physical events is an interactionist dualism (Chalmers, "Conscious" p. 162). Chalmers leaves open the possibility of some causal relevance for conscious experience (Chalmers, "Conscious" p. 160), but does not accept interactionist dualism because he regards conscious experience as "explanatorily irrelevant to our claims and judgments about consciousness" (Chalmers, "Conscious" p. 177). He calls this explanatory irrelevance the paradox of phenomenal judgment (Chalmers, "Conscious" p. 177).
Consider the analogy of a very simple computer with a lot of memory. The computer's CPU has just enough instructions to be able to calculate square roots to any precision that its memory capacity will permit. No answers are programmed into the computer, but a simple procedure for computing square roots is programmed into it. The computer then proceeds to compute the square root of 2. During computation, the computer does not accept any input at all, and it has a single digit output, so it outputs the resulting digits serially, one at a time. When computation begins, the computer has a simple organization: simple hardware, a simple program, and a large but blank memory. As computation progresses, the computer's organization becomes progressively more complex because it is exploring an infinitely complex structure without the benefit of any input information. The structure does not physically interact with the computer (either directly or indirectly), yet more and more of the structure is being registered in the computer's memory and serial output. The computer can keep exploring the infinitely complex structure until it runs out of memory. Had the computer been calculating a rational number, it could have used a fixed amount of memory to output digits of the solution indefinitely since the output digits would have been in a repeating pattern. In computing the square root of 2, the computer does not have the benefit of a repeating pattern, so it must store an ever increasing amount of information to keep the computation going. One might expect the structure of the solution to the square root of 2 to be finitely complex because the program for computing it is finite. However, the program for computing the square root of 2 is not the structure of the solution itself, but only part of a means of exploring the structure. The structure that the computer is exploring is infinitely longer than it could ever explore with its finite resources. But the computer does register a significant part of the structure. The structure is not a physical entity, but a mathematical one. The computer explores this nonphysical, abstract entity by purely physical means.
Now if a computer can register a significant part of an infinitely complex mathematical entity by purely physical means, the physical brain might register nonphysical conscious experience by purely physical means, too. Just as the infinitely complex mathematical structure does not physically interact with the computer, the brain might register conscious experience without physically interacting with any sort of ghost in the machine. Though conscious experience and mathematical entities have radically different natures, this analogy might be helpful in trying to make sense of the paradox of phenomenal judgment.
Of course, the structure of the numerical solution to the square root of 2 is a matter of logical necessity, whereas a person's judgment about having conscious experience does not logically necessitate the presence of conscious experience in the person. But the analogy only concerns their common registration of nonphysical entities in physical systems, not their contingency or necessity. One might object that the computer's registration of the numerical solution has a mathematical causal explanation. But in that case the actual structure of the numerical solution is irrelevant to any mathematical causal explanation of the physical processes by which it is registered, for it was not derived by programming the numerical solution itself into the computer.
If Chalmers is right that conscious experience is explanatorily irrelevant to our claims and judgments about conscious experience, this presents no problem for the personal soul hypothesis. For nothing in the personal soul hypothesis (as I have defined it) requires anything about a person's conscious experience to have an effect on the outcome of any physical process. So the personal soul hypothesis is as compatible with the explanatory irrelevance of conscious experience as it is with interactionist dualism.
9.4 Fading and Dancing Qualia
Chalmers supports his principle of organizational invariance with two thought experiments. In the fading qualia thought experiment (Chalmers, "Conscious" pp. 253-263), silicon circuits gradually replace neurons in a way that fully preserves fine-grained functionality. He demonstrates the absurdity of qualia progressively fading away as electronic circuits progressively replace brain cells. In his dancing qualia thought experiment (Chalmers, "Conscious" pp. 266-274), part of the brain has all its output connections to the remainder of the biological system interrupted by switches that switch between it and a functionally equivalent electronic system. Chalmers then shows how it would be absurd for conscious experience to dance between one kind of qualia and another as the switches alternate between the organic brain and the electronic functional isomorph.
Neither thought experiment presents a serious problem for the personal soul hypothesis. Suppose it is possible for electronic circuits to replace organic neurons and support conscious experience. There would need to be some integrating process to make the replacement. The electronic circuits could become associated with the same personally unique phenomenon as is the organic brain through the process of integration. The multiverse analogy is helpful here. Victor Stenger discusses the possibility of other universes in which hydrogen atoms exist, but their physical properties are different due to a difference in the physical constants of the universes (Stenger, "Natural"). For simplicity, consider an electron from one such other universe. In our universe, an electron has a free space rest mass of 4.185*10-23*(h_bar*c/G)½. This number would be different in another universe because it would have a different Higgs field. Suppose an electron from the other universe could be transported into our universe. The electron would no longer have the free space rest mass that it had in its universe of origin, but it would have the same free space rest mass as our universe's electrons because it would now be in the Higgs field of our universe. Analogously, if foreign material is integrated into a person's brain and takes on a particular role in the person's conscious experiences, then the conscious experiences for which it has a role could have the same personally unique property no matter what its former roles may have been. If silicon chips could support conscious experience, then their integration with an organic brain could result in their associated conscious experiences having the same personally unique property as the organic brain into which they were integrated.
There are some challenges for the dancing qualia experiment. For one, the entire organic brain must remain active even when part of it is switched out. Although its output connections are switched out, the portion of organic brain being switched is always receiving precisely the same inputs and is changing states exactly as it would if its outputs were still connected. If this were not so, there would be a discontinuity when it is switched back in. So conscious experience continues in the switched portion of the organic brain when it is switched out in the same way that it would continue if it were switched in. Therefore, the switching would have no effect on conscious experience regardless of whether conscious experience associated with the backup circuit is identical, different, or absent. Furthermore, if brain function is not deterministic, then the internal states of the organic brain and the backup circuit would diverge despite identical inputs and identical initial states. This divergence would result in a discontinuity when switching. Both of these points pose problems for the thought experiment of dancing qualia but not for that of fading qualia.
10. Incompatibility of Organizational Invariance with the Personal Soul Hypothesis
Chalmers' assumption that conscious experience naturally supervenes on physical features of our world leaves no room for anything nonphysical to determine anything particular about a conscious experience. The natural supervenience thesis thus excludes the personal soul hypothesis because the latter entails that a nonphysical property determines something essential about a person's conscious experience. The principle of organizational invariance goes even further, allowing a wide variety of physical implementations of a functional process to produce exactly the same conscious experience. Chalmers strong AI claim goes further still, allowing an appropriately programmed digital computer to generate any given conscious experience.
But on the personal soul hypothesis, the physical features of a person are not sufficient to determine everything about a person's conscious experience. There would be a range of physical conditions necessary to support the association of a soul with processes in a physical system, but no set of physical conditions would determine everything about a person's conscious experience. A person in this world and a twin in a counterfactual world could be physically identical and yet have a fundamental difference in their conscious experience due to a difference in some personally unique, nonphysical property.
11. Possible Loopholes for the Personal Soul Hypothesis in Chalmers' Principles
11.1 A Possible Loophole in the Principle of Organizational Invariance
If a personally unique property of conscious experience could be considered part of the fine-grained functional organization that determines behavioral capacity (Chalmers, "Conscious" p. 248), then the personal soul hypothesis would not conflict with Chalmers' principle of organizational invariance. If a person's expressed belief in the personal soul hypothesis is a consequence of the personal soul's existence, then the personal soul would be a factor in determining the behavioral capacity for such a belief. It is deeply implausible that belief in conscious experience is due to some kind of psychological illusion, for conscious experience is "at the very center of our epistemic universe" (Chalmers, "Conscious" p. 74). Nevertheless, it is plausible that belief in the personal soul hypothesis could result from an illusion from the unified (Chalmers, "Conscious" p. 306, 309) and deeply first-person (Chalmers, "Conscious" p. 199) nature of conscious experience, as well as the psychological sense of self (Dennett, "Consciousness" pp. 412-430). This makes it more challenging both to keep this loophole open, and to defend the personal soul hypothesis itself. To keep this loophole open, we would need to find some reliable sort of behavioral difference between persons that could not be attributed to any psychological cause. Even if belief in a personal soul is a consequence of the existence of a personal soul, but the personal soul does not determine any behavioral difference between persons, then the personal soul hypothesis would still conflict with Chalmers' principle of organizational invariance. Chalmers considers the possibility that behavioral acts are not purely physical (Chalmers, "Conscious" p. 183). But he points out that conscious experience would then still be explanatorily irrelevant to a person's physical behavioral processes. However, this explanatory irrelevance does not rule out the possibility that some feature of a person's conscious experience could be a factor in nonphysical components of a person's behavior.
Chalmers' view that conscious experience naturally supervenes on physical processes is more problematic for this loophole than is explanatory irrelevance. According to the natural supervenience thesis, all particular facts about any conscious experience are fully determined by physical facts. This leaves no room for anything nonphysical to determine any particular facts about a conscious experience. Even if behavioral acts are not purely physical, there would still be nothing nonphysical in a person's fine-grained functional organization that would be a factor in any particular facts about a person's conscious experience. The principle of organizational invariance could thus be restated: "given any system that has conscious experiences, then any system that has the same fine-grained [physically] functional organization will have qualitatively identical experiences" (Chalmers, "Conscious" p. 249).
11.2 A Possible Loophole in Natural Supervenience
To keep open the aforementioned loophole for the personal soul hypothesis in Chalmers' principle of organizational invariance, one would also have to find a similar loophole in his view that conscious experience naturally supervenes on physical features of the world. Such an additional loophole might exist if a person's unique property of conscious experience were actually a physical feature of our world. But Chalmers rejects the idea that conscious experience is a physical feature of the world on the following grounds: "Experience is not a fundamental property that physicists need to posit in their theory of the external world; physics forms a closed, consistent theory even without experience" (Chalmers, "Conscious" p. 128). He also rejects the possibility that conscious experience might be included in some extended or future physics: "it seems that all physics will ever entail is more structure and dynamics, which (unless one of the other reductive options is embraced) will never entail the existence of experience" (Chalmers, "Conscious" p. 163). However, there might be physical laws of conscious experience that are inaccessible to physical investigation (Chalmers, "Conscious" pp. 134-136) that need not be considered when explaining anything other than conscious experience.
Recall that Chalmers thinks that conscious experience correlates with physical processes according to a yet-to-be-discovered system of psychophysical laws, but does not consider conscious experience itself to be physical because of the logical possibility of a zombie world that is physically identical to ours (Chalmers, "Conscious" p. 123). However, a phenomenal zombie could be logically possible given known physics, but logically impossible on an extended physics where conscious experience is a physical feature of the person. The characterization of conscious experience as physical is what makes phenomenal zombies logically impossible on such physics. Analogously, a unique solution to the problem of x3 = 1 is possible in real number mathematics, but it becomes impossible in complex number mathematics, because it has three solutions there. Chalmers is willing to accommodate the possibility that there are intrinsic properties inaccessible to physical investigation by replacing "physical" and "nonphysical" properties with "accessible" and "hidden" physical properties in his description of his naturalistic dualism (Chalmers, "Conscious" p. 136). In doing so, Chalmers leaves open the possibility that some "hidden" physical property could be considered part of the fine-grained functional organization that determines behavioral capacity (including "hidden" behavior).
12. Deeper Reality
There are no provisions in present-day physical science for the personal soul hypothesis. From a scientific perspective, it is difficult to see why conscious experience with some personally unique property would be in an intimate association with a particular physical system for many years. However, present-day science offers no explanation for the existence of conscious experience in the first place, regardless of whether or not the personal soul hypothesis is true. Both the personal soul hypothesis and the existence of conscious experience could be supported by a "deeper" view of reality, however, where reality encompasses more than just the physical. Though Chalmers does not accept the personal soul hypothesis, he does affirm that reality is deeper than the physical alone in the following three statements:
Eliminativism about conscious experience is an unreasonable position only because of our own acquaintance with it. If it were not for this direct knowledge, consciousness could go the way of the vital spirit. To put it another way, there is an epistemic asymmetry in our knowledge of consciousness that is not present in our knowledge of other phenomena. Our knowledge that conscious experience exists derives primarily from our own case, with external evidence playing at best a secondary role. (Chalmers, "Conscious" p. 102)
Any account given in purely physical terms will suffer from the same problem. It will ultimately be given in terms of the structural and dynamical properties of physical processes, and no matter how sophisticated such an account is, it will yield only more structure and dynamics. While this is enough to handle most natural phenomena, the problem of consciousness goes beyond any problem about the explanation of structure and function, so a new sort of explanation is needed. (Chalmers, "Conscious" p. 121)
The existence of consciousness will always be a further fact relative to structural and dynamic facts, and so will always be unexplained by a [purely] physical account. (Chalmers, "Conscious" pp. 121-122)
Like the personal soul hypothesis, Chalmers' view holds that conscious experience is a further fact that is not explained by any physical facts. But his principle of organizational invariance is a significant further assumption about conscious experience that excludes the personal soul hypothesis. The personal soul hypothesis makes the further assumption that there is a unique conscious experience for each person.
A deeper view of reality requires neither an abandonment of, nor any exceptions to, physical principles. Chalmers thinks that the physical world is "more or less causally closed" (Chalmers, "Conscious" p. 125). But he also thinks that "[c]onsciousness is a feature of the world over and above the physical features of the world" (Chalmers, "Conscious" p. 125). Nothing in these two ideas conflicts with the personal soul hypothesis. A unique property of conscious experience for each person would be a feature of the world over and above its physical features, but it need not violate any physical laws.
Except for the two possible loopholes discussed above, there is no room in Chalmers' principle of organizational invariance for the personal soul hypothesis. However, there is plenty of room in the personal soul hypothesis for psychophysical laws. If physical facts do not determine everything about a person's conscious experience (i.e., if Chalmers' natural supervenience thesis is false), they could nevertheless still determine a large part of it. Very important facts about the quality of conscious experience may be independent of any soul property. A successful, more or less closed system of psychophysical laws might be developed without regard to the personal soul hypothesis, but its success wouldn't rule out the personal soul hypothesis. Just as Chalmers hopes to develop a system of psychophysical laws beyond the more or less causally closed system of physical laws, there could also be—consistent with the principle of organizational invariance—laws relevant to the personal soul hypothesis beyond a more or less closed system of psychophysical laws. In my view, the personal soul hypothesis is not about one person's red being another person's blue (as in the inverted qualia thought experiment). It is a far deeper and fundamental matter of personhood.
 Chalmers distinguishes between two main kinds of supervenience, logical and natural supervenience (Chalmers, "Conscious" p. 34-38). In logical supervenience, the "low-level" properties of a thing determine its "high-level" properties by logical necessity. In natural supervenience, the low-level properties of a thing contingently determine its high-level properties by nomic or natural necessity—that is, due to laws of nature. In maintaining that facts about conscious experience do not logically supervene on physical facts, Chalmers rejects strict materialist views of conscious experience (Chalmers, "Conscious" p. 123). However, he believes that facts about conscious experience naturally supervene on physical facts (Chalmers, "Conscious" pp. 124-129).
 The idea of a soul inhabiting a particular place needs some clarification here. I do not mean to imply that the soul is some kind of substance that fits into a particular volume. The soul is better thought of as a zone of association than as a place of physical occupancy. The personal soul hypothesis proposes that all conscious experiences associated with a particular person will have some common property that is unique to the person. Through the soul's association with processes in the person's biological system, the person's conscious experiences would acquire extrinsic qualities like blueness, painfulness, and warmth, along with structure, but the basic soul would have an existence of its own, independent of the physical system with which it is associated. By contrast, conscious experience with no personally intrinsic property would have no independent existence, but would arise from processes within a physical system. Since the conscious experiences that we are familiar with are intimately associated with physical processes in a person's head, the person's head may be viewed as a habitation of the person's conscious experiences. However, the personal soul hypothesis does not limit a person's soul to physical processes in the head. It also allows the existence of conscious experiences with the same personal property unassociated with any physical processes at all. Though it is difficult to conceive how such unassociated conscious experiences could have any extrinsic qualities, or what conscious experience would be like with no extrinsic qualities or structure at all, it nevertheless must be left open as a possibility.
 I would like to thank Keith Augustine and an anonymous referee for providing valuable comments on this paper.
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