Hylemorphic dualism is the approach to the mind-body problem taken by Aquinas and the Thomist tradition more generally. (The label may have been coined by David Oderberg, who defends the view in an important paper and in his book Real Essentialism. “Hylemorphic” is sometimes spelled “hylomorphic,” though the former spelling is arguably preferable since it is closer to the Greek root hyle.) The view holds both that the soul is the substantial form of the living human body (that is the “hylemorphic” part) and that it is unique among the forms of material things in being subsistent, that is, capable of surviving beyond the death of the body (that is the “dualism” part). Our friend Bill Vallicella has recently put forward the following criticism of the view:
How can a substantial form exist apart from that of which it is the form? Is it not necessarily tied to that of which it is the form? After all, it is so tied in the case of non-humans like Fido. Fido is a composite the components of which cannot exist on their own. Why should it be any different in the case of the human soul if the human soul is indeed the form of the human body?
The problem here, in short, is that there is a tension between soul as substantial form and soul as substantial subsistent form. Ontologically, one wants to protest, a form is not the sort of entity that could be subsistent. Necessarily, a form is a form of that of which it is the form. But a subsistent form is possibly such as to exist apart from that of which it is the form. These propositions cannot both be true.
The problem here, in short, is that there is a tension between soul as substantial form and soul as substantial subsistent form. Ontologically, one wants to protest, a form is not the sort of entity that could be subsistent. Necessarily, a form is a form of that of which it is the form. But a subsistent form is possibly such as to exist apart from that of which it is the form. These propositions cannot both be true.
I find it hard to resist the suspicion that what Aquinas has done is implanted Christian elements into the foreign soil of Aristotelianism. Christianity requires that the soul be capable of independent existence. But no form, by its very nature as form, is capable of independent existence. Simply to make an exception in the case of the human soul is wholly unmotivated and ad hoc and inconsistent with hylomorphic ontology.
Naturally, since I am a hylemorphic dualist, I completely disagree with Bill here. Let’s start with the last charge -- that hylemorphic dualism “make[s] an exception in the case of the human soul [that] is wholly unmotivated and ad hoc and inconsistent with hylomorphic ontology.” That the view is not “unmotivated and ad hoc” is easily shown. Bill himself would surely acknowledge that there are serious philosophical arguments for hylemorphism, even if he doesn’t accept that view himself. He would also acknowledge that there are serious philosophical arguments for dualism, a view he is sympathetic with. But then he should also acknowledge that someone could find both sorts of arguments convincing. And in that case he should acknowledge that someone could have good philosophical reasons for thinking that there must be some way to combine hylemorphism and dualism.
That, I submit, is precisely the position Aquinas finds himself in. As an Aristotelian, he is convinced that the human soul is the form of the living human body. It is therefore responsible for all the various human capacities -- nutrition, reproduction, growth, sensation, appetite, locomotion, intellect, and volition -- in just the way the souls of plants and non-human animals are responsible for their capacities. But Aquinas is also convinced that our purely intellectual capacities cannot have a corporeal organ. The reason is that he endorses philosophical arguments for the immateriality of the intellect of the sort that go back to Plato and Aristotle. That much gives him grounds for concluding that the soul carries out immaterial operations alongside its corporeal ones. Add to this the (independently motivated) Scholastic thesis that agere sequitur esse -- that “action follows being,” so that the way a thing acts reflects the manner in which it exists -- and we have grounds for concluding that, though the soul is the form of the body, it must in some way have a kind of subsistent immaterial existence. The view might seem odd, but it is hardly unmotivated or ad hoc. On the contrary, it is a natural way of trying to reconcile two theses that Bill himself would acknowledge to have serious philosophical arguments in their favor.
Nor, contrary to what Bill implies, is Aquinas somehow departing radically from Aristotle. For Aristotle too was committed both to hylemorphism and to the view that the intellect is immaterial -- indeed, to the view that the active intellect is immortal. To be sure, that does not by itself show that Aristotle’s views are identical to or entail Aquinas’s; the Averroists took Aristotle’s position in a very different direction, and contemporary commentators often find it simply puzzling. But the reason they do -- namely, that it seems odd to say both that the soul is the form of the body and that one of its capacities is somehow separable from the body -- is similar to the reason Bill finds Aquinas’s position puzzling. Needless to say, Aristotle had no Christian theological ax to grind; he was simply following the philosophical arguments where they led. There is no reason to accuse Aquinas of doing anything different, and it is hardly unreasonable to suggest that the way to harmonize the various aspects of Aristotle’s position is the way Aquinas does. That does not mean that one might not still question whether Aquinas’s position is ultimately coherent (as Bill does), or criticize it on other grounds. But the charge that it is “wholly unmotivated and ad hoc” -- a piece of Christian apologetics with no independent philosophical rationale -- is, I think, completely unwarranted.
Now, does Aquinas’s dualism cohere with his hylemorphism? Bill thinks not, but here too I think he has failed to make his case. Let’s note first that there is nothing in hylemorphism that requires that we deny that a form per se can have an existence apart from matter. Aristotle’s opposition to Platonism might seem to rule this out, but it doesn’t. What Aristotelianism rules out is that universals can exist both apart from their instances and apart from any mind. But when Aquinas says that certain forms exist without matter -- the human soul, or an angel -- he is not talking about universals existing apart from matter. Nor is he even talking about a form by itself existing apart from matter, but rather a form plus an act of existing. Hence he is talking about concrete particulars, albeit immaterial ones. (Aristotle himself, who knew a thing or two about hylemorphism, allowed for immaterial things -- the “Intelligences” which he took to move the heavenly spheres.)
So, there is nothing necessarily un-Aristotelian in the notion of a form without matter. But what about the form of a material thing? The soul is, for Aquinas, the form of the body. So how could it possibly exist apart from the body? Bill asks why things should be any different with human beings than they are with Fido. But Aquinas is quite clear about the answer to that question: The difference is that the human soul carries out immaterial operations (i.e. intellectual ones) while a dog’s soul does not. And if it operates apart from matter and agere sequitur esse, then it must subsist apart from matter. True, it would not subsist as a complete substance since (qua form) it is only part of a complete substance. But it would subsist as an incomplete substance, like a severed hand which subsists at least for a time apart from the body (as can be seen from the fact that the hand can be reattached).
This last example tells us what is wrong with Bill’s further objection:
Necessarily, a form is a form of that of which it is the form. But a subsistent form is possibly such as to exist apart from that of which it is the form. These propositions cannot both be true.
That they can both be true can be seen when we keep in mind how Aristotelians understand concepts like necessity, possibility, essence, and the like. Suppose we say that it follows from the nature or essence of a dog that it has four legs. Does that mean every single dog necessarily has four legs? No, because a given dog might have lost a leg in an accident, or failed to develop all four legs due to some genetic defect, or (if only recently conceived and still in the womb) may simply not yet have developed all four legs. What it does mean is rather that a mature dog in its normal state will necessarily have four legs. As Michael Thompson and Philippa Foot have emphasized, “Aristotelian categoricals” of the form S’s are F convey a norm and are not accurately represented as either existential or universal statements of the sort familiar to modern logicians. “Dogs have four legs” is not saying “There is at least one dog, and it has four legs” and neither is it saying “For everything that is a dog, it is four legged.” It is saying that the typical dog, the normal (mature) dog, has four legs.
Similarly, to say “Human souls are associated with bodies” is to say that the human soul in its normal state is associated with its body, just like the human hand in its normal state is associated with its body. But it doesn’t follow that it cannot exist apart from the body, any more than it follows that the hand (at least while its tissues are still alive) can exist apart from the body. And again, the reason this is possible with the human soul and not with Fido’s soul is that the human soul, unlike Fido’s soul, carries out immaterial operations even when it is associated with the body.
As Gyula Klima emphasizes in his article “Man = Body + Soul: Aquinas’s Arithmetic of Human Nature,” when trying to understand Aquinas’s conception of the soul, we need to keep in mind the more general metaphysical and semantic framework within which he is working. In particular, we need to keep in mind his doctrine of analogical predication:
[I]f we recognize the analogical character of the predication of the notion of being with respect to the whole and with respect to its essential parts, it should come as no surprise that body and soul in the exclusive senses of these terms are said to be one being, in the primary sense of the term, yet they can be said to be two beings in the derivative sense in which distinct parts of a whole can be said to be beings.
To be sure, there are some things that can properly be predicated only of the composite of soul and body together, and not to either soul or body by itself:
[S]ince having power and action can attach properly only to a being in the primary sense, the problem of interaction between body and soul… cannot arise, for both the actions and the corresponding powers will still belong only to the unified whole, and not to either of the parts.
But there is a qualification:
This has to be the case, at least, unless there is some action which can properly be said to belong only to one of them, in which case that part will also have to be regarded as a being not only in the sense in a which a part is a being, but also in the sense in which the whole is.
But this is precisely the point Aquinas makes with respect to the unique case of the human soul in his proof for its immortality… [when he argues that] understanding is the act of the intellective soul alone…
Here as elsewhere, we will fail properly to understand Aquinas unless we see how thoroughly he rejects the assumptions that inform the thinking of most modern philosophers. As with his views on ethics and the arguments for the existence of God, so too with his views on the mind-body problem, Aquinas’s position rests on a radically different understanding of the nature of causation, substance, essence, modality, and other basic metaphysical notions. He and other ancient and medieval philosophers aren’t merely proposing a novel move in the game being played by contemporary philosophers of various stripes. They are challenging the game itself.
