Things have been a bit slow here at SO the past couple of weeks with Jeff and others spending time bashing Donald Trump (an entirely worthwhile enterprise), so I will try to enliven things a bit. Although the notion plays no part in neuroscience, cognitive science, cognitive psychology, or any other of the sciences of the brain and mind—where the regulative assumption has long been that the brain is sufficient for all mental functions—souls still have their defenders. By “soul” I mean a simple, spiritual substance understood in the Cartesian sense, i.e. as the center of consciousness and personality that is the essential self and which, though it interacts with the physical, is not itself part of the physical world. The soul is therefore a supernatural entity that, during a person’s earthly life is closely connected with one particular physical body, but which lives on after physical death.

The postulation of such a soul is (or at one time was) plausible, and the idea continues to offer many advantages. Most obviously, it offers hope in the face of the most basic and primal of human fears, the fear of death. Since it is not part of the physical world, souls are not subject to the physical forces that lead to the decay and ultimate dissolution of the body; rather they are essentially immortal. Equally obviously, the postulation of souls is friendly to a theistic worldview. God, and infinite spirit, would plausibly create many finite spirits, including some that, for whatever reason, are temporarily bound to material bodies. Further, the metaphysically spiritual souls are also spiritual in another sense, namely that the soul is the part of the human person that longs to reunite with God—as is sung so beautifully in Mahler’s “Resurrection” Symphony. Finally, the idea, which is intuitive for some, that we have a will is an unmoved mover—capable of actions that are determined by nothing except our own intrinsic agency—is supported by the attribution of the power of choice to a nonphysical entity. Such an entity, qua nonphysical, is not subject to the causal laws of the physical cosmos, and so can serve as an uncaused cause. Christian philosophers Stewart Goetz and Charles Taliafero argue for the existence of souls largely in terms of our allegedly intuitive first-person experiences of free choice.

Here I offer three reasons for the non-existence of souls as defined above. First, though, in any such discussion, it is helpful to determine on whom the burden of proof should fall. Is the default presumption to be that there are souls or that matter, suitably organized, is sufficient for mental activity? I claim that the burden of proof falls entirely on those who support the existence of spiritual souls. Why? Well, first, let us consider what, I presume, is known to everyone: We know that certain configurations of matter—those configurations we refer to as “human beings,” for instance—are capable of performing mental functions. They think, feel, perceive, imagine, desire, will, believe, and so forth. If, then, certain configurations of matter can perform mental functions and possess mental properties, the parsimonious, spontaneous, and natural assumption would be that matter, when organized in suitable ways, can perform mental functions and possess mental properties.

It seems perverse to make the opposite assumption, namely that material beings cannot think, and that therefore their mental functions and properties must be due to the operation of something non-physical, a soul perhaps. To make this last assumption would seem to be a bizarre instance of an a priori prejudice. The proper starting point therefore appears to be that material beings are capable of doing whatever we observe them doing, not with the gratuitous and a priori assumption that they cannot. Therefore, the burden of proof should be on those who say that matter is incapable of mental functions or of possessing mental properties, and that these must instead be due to something non-material.

The second reason for putting the burden of proof on the soul-theorist is that, surely, by now, the heuristic assumptions of neuroscience have gained some degree of authority. As I mentioned earlier, a regulative assumption of all the sciences that study mind and brain is that the brain is sufficient for all mental activity. Perhaps, as David Chalmers famously argued, we may never solve the “hard problem,” that is, to understand exactly why some physical events should cause mental events. Still, Chalmers takes for granted that physical events cause mental events, so he accepts the regulative assumption. When a program of inquiry has produced hard, reproducible, and important results, as has neuroscience, then this would warrant prima facie acceptance of the heuristic principles that have guided such research. The burden of proof should therefore fall on those who reject the assumption of the sufficiency of the brain and instead invoke non-physical entities.

Therefore, the burden of proof is on those who claim that souls exist. I now note three problems that anyone advancing a theory of souls must address and say why I think that these problems are intractable:

1) The old problem that will never go away—interaction. Princess Elizabeth of Bohemia was the first to raise the interaction problem in a letter to Descartes. The problem is simply stated, namely, how do we conceive of the interaction between two types of entity as metaphysically disparate as soul and matter? It is good enough for ghost stories to imagine poltergeists upsetting the furniture, but, really, how do we achieve any degree of conceptual clarity? How does a putative entity with purely mental capacities initiate and participate in a series of physical events? By definition, souls have no physical properties, and there seems no clear way to conceive of how something with no physical predicates (not even location in space) can enter into causal relations with physical events.

Descartes famously had no plausible answer for the princess. Today, soul-theorists often just bite the bullet and offer a tu quoque. They concede that, indeed, we do not know how spirit could interact with matter, but, they argue, neither do we know how matter interacts with matter (tu quoque!). That is, at rock bottom everybody postulates brute facts about what does happen, with no deeper understanding of why it happens. Why, for instance, do like electrical charges repel and opposite charges attract? Well, maybe when we get down to that level of explanation all we can say is that that is how things are and that there is no deeper reason why. In that case, the soul-theorist will argue, at rock bottom, and whatever our ontology, we all have to posit basic entities with sets of irreducible and inexplicable powers and liabilities. At the deepest level shit happens, and that is all there is to it. In this case, soul-theory on an even level with physical theory. Both postulate brute facts of interaction that neither can explain at any deeper level, so why the prejudice against souls?

Such a reply is misleading at best. Even if we reach a brutally factual rock bottom in physical theory, where we just have to postulate fundamental entities and forces that turn out not to be further explicable, such brute facts lie at the end of a very long chain of deeply satisfactory explanations. Hume somewhere asserts that we will probably never understand why bread nourishes. Ah, but we do. We have for quite some time understood in very considerable detail how mitochondria break down complex carbohydrates and, via the chemical pathway known as the Krebs cycle, provide energy at the cellular level. Indeed, molecular biology is full of extremely detailed explanations of physiological processes that tell us why they happen just as they do. At a more basic level, nuclear physics can explain in detail why nuclear weapons bang so prodigiously. In innumerable cases, we do not say just that shit happens, but precisely why it happens. Even at the fundamental level of photons and electrons, theories such as Quantum Electrodynamics provide much well-confirmed and definite information about how and why things happen.

Thus, the brute facts of physical theory may be there, but they are down very deep. With soul-theory, the incomprehensibility is right up front and on top. Your inquiry immediately hits a wall. “How do I think?” “Your soul does it.” “How does it do it?” “Let me explain: Shut up!” Sorry, but I am not just being flippant here. It matters where you put your brute facts. Mystery-mongers take you right to the occult, “explaining” in terms of astrological influences, or hexes, or psi, or chakras, or qi, or whatever. Honestly, the postulation of souls just seems to be another appeal to the occult. Neuroscience thinks we can go deeper without any such paranormal postulates, and we can.

2) The self is not simple or permanent. Soul-theory holds that your soul is you. That is, from the moment you acquire a soul (this bit is murky), that soul is your essential self, and it remains you throughout your life—and after. The soul is therefore a simple, spiritual essence, an abiding self that remains a unified whole throughout the vagaries of your mental being as you go through all the stages of life. You may think and feel very differently at sixty than you did at twenty, but that thinker will still be you, a permanent, indivisible entity that makes you you.

What, though, is the self? Surely, it is something complex, not simple. Speaking personally, what I call my “self” seems to be, first, a nexus of fleeting thoughts, feelings, and perceptions. The thought that student X seems to be failing my class, a desire for French fries, a pang over a long ago faux pas, and a momentary dread of tomorrow’s faculty meeting, all can pass through my mind then be gone, actors on a Cartesian stage as Daniel Dennett calls it. Then there is my “true” or “real” self, those values, beliefs, and desires that are deep and longstanding and with which I identify, for instance, my deepest philosophical and political convictions. There are also characteristic capacities and incapacities such as my ability to conduct a class on Plato’s Republic, but my inability to appreciate certain types of music or art (I just cannot like Bartok, however patiently his aficionados explain his virtues.). Then there are certain long-term personality traits that you might carry your whole life, including good ones like a sense of humor and bad ones like a quick temper. Then there is the narrative self, the stories I tell about my life and how and why I think it unfolded as it did.

The “self” therefore seems multifaceted and multilayered, a confederation of heterogeneous traits, experiences, virtues, vices, quirks, abilities, stories, and so forth—the million-and-one features that, in their endless variety of combinations, make each of us unique individuals. Further, the self clearly seems to change over time. I sincerely hope that I am not now the lazy, irresponsible, immature person I was at twenty. True, some traits stay with you, but to say “I am not the same person I was thirty years ago” can be literally true. Our basic values, beliefs, and desires can change, sometimes profoundly. It therefore seems arbitrary and dogmatic to insist that, despite the apparent multiplicity and mutability of the self, that it is, all along, just one simple, permanent thing. In the most tragic cases, such as the final stages of Alzheimer’s disease, the self can be lost entirely. Soul-theorists say that in such cases the true self is still there, but that the illness prevents it from expressing itself. This seems entirely ad hoc to me and no more plausible than saying that an over-the-hill quarterback has just as good an arm as he ever had, but age prevents him from displaying it.

The above considerations point to an even deeper problem. The self is anything but a thing. As noted, the self is defined in complex terms of our experiences, character, propensities, abilities, personalities, narratives, and so forth. In that case, as Ryle observed long ago, it is a category mistake to view the self as a thing, as substantial entity. By analogy, a culture would similarly require a complex definition in terms of traditions, beliefs, myths, artifacts, customs, and practices. A culture is obviously not a thing, and, equally obviously, neither is a self.

3) Who needs souls? Seriously, who does need them? If souls are necessary for mental operations, what about the mental operations of animals? The Cartesians infamously got around this problem by claiming that nonhuman animals were just machines. We now have a considerable body of information about the intellectual and emotional capacities of nonhuman animals. Copious experimental evidence confirms that animals have a diverse set of faculties for reasoning and problem solving. Some animals even display “moral” emotions such as grief and empathy. So, do animals have souls? If we say “no,” then we need to give a principled, i.e. non-arbitrary, answer to the question of just what level of ability or what types of functions require souls and why this is so. If, as Darwin claimed, the differences between human and animal minds are differences of degree rather than kind, where in that continuum of abilities do we say that brains are no longer sufficient and that souls are required for a higher level of aptitude? Any answer would seem to be arbitrary.

Well, then, maybe we should say that nonhuman animals also might have souls. However, this only raises the same problem anew. Animals show a broad range of mental functions from none at all to quite sophisticated competencies. At what point do we say that here, just here is where souls are needed and brains are not enough? With which animals do we say that their mental functions are so sophisticated that they must have souls? Bonobos? Monkeys? Cats? Snakes? Frogs? Oysters? Again, any answer would seem to be arbitrary. Finally, we know that Homo sapiens is the culmination of a long period of evolutionary development. If humans have souls, what about their ancestors? Where in the line of our ancestors did we acquire souls? Did Homo neanderthalensis have souls? What about Homo erectus? Homo habilis? What about Australopithecus afarensis? Will Lucy be in heaven? Once more, any answer would seem to be arbitrary.

The point of the above queries is this: Where do we drive the golden spike indicating that from this point brains are insufficient? How do we give a principled and non-arbitrary answer? If no such principled answer is possible, and I submit that none is, the whole rationale for a soul seems to disappear.

I imagine that soul-theorists would admit that they have no clear answer to where to drive the “golden spike,” but that, while perhaps embarrassing, they are prepared to grin and bear it, recognizing that physicalists have their own embarrassments, q.v. the “hard problem” mentioned earlier. However, the “hard problem,” if it is a legitimate issue, is just as intractable for soul-theorists as for physicalists. It is no clearer how a putative soul generates consciousness than how brains do so. If the reply is that a soul just is consciousness, then “soul” becomes a redundancy; attributing a soul will just be a useless way of saying that some things are conscious.

It is time for souls to go the way of the other useless supernatural baggage—like witches, wizards, familiar spirits, hexes, djinn, banshees, and things that go bump in the night.