One must always be cautious when trying to relate Aquinas’s position on some philosophical issue to the options familiar to contemporary academic philosophers. Sometimes he is not addressing quite the same questions they are, even when he seems to be. Sometimes he does not use key terms in the same ways they do. And he is working with a general metaphysical picture of the world – in particular, a picture of the nature of substance, essence, causation, matter, and other fundamental notions – that is radically different from the options familiar to contemporary philosophers, in ways the latter often do not realize.
Having said that, let me propose tentatively that there is a way that contemporary philosophers of mind might naturally fit Aquinas’s position on the mind-body problem into the framework of options they are familiar with – a way that is surprisingly neglected by them, even though it is pretty clearly implied by the way they tend to classify the options.
The usual classification goes something like this. Descartes got the modern discussion of the mind-body problem going by putting forward his famous version of substance dualism. For Descartes, there are two irreducibly different kinds of substance. One of them is matter, conceived of as res extensa or extended substance. The other is mind, conceived of as res cogitans or thinking substance. The nature of each is entirely exhausted by extension and thought, respectively, and neither has any of the attributes of the other. Res extensa doesn’t merely haveextension, but just is pure extension, utterly devoid of thought or consciousness. Res cogitans doesn’t merely havethought, but just is pure thought, utterly devoid of extension.
Either one of these substances could exist in the absence of the other. (That’s why each counts as a substance. Each has an independent or freestanding existence relative to the other.) A res extensa by itself and in the absence of a res cogitans would, even if it had all the physical and behavioral characteristics of a human body, be as devoid of thought and consciousness as a stone or a table. It would be a “zombie,” in the sense familiar in contemporary philosophy of mind. A res cogitans by itself and in the absence of a res extensa would be essentially like an angelic intellect. A human being is a composite of these two kinds of substance, the result of their getting into a relation of efficient causal interaction.
Later modern philosophers do not all entirely agree with Descartes’ characterization of matter or of mind, but, to oversimplify a bit, they can be understood as essentially beginning with Descartes’ bifurcation and then modifying it in different ways. One way to modify it is to keep res cogitans but deny that res extensa is real. All that exist, on this view, are minds and their ideas, and tables, chairs, rocks, trees, and the like are really just collections of ideas. Only mental substances and mental attributes are real, and physical objects are real only in the sense that they can be reduced toideas, which are mental attributes. This is the idealism spelled out in different ways by thinkers like Leibniz and Berkeley.
Another way to modify Descartes’ picture is to keep res extensa instead and get rid of res cogitans. On this approach, all that exist are material substances and material attributes, and thoughts, experiences, and the like are, if they are real at all, really just material attributes of some sort (neurological attributes, or computational attributes, or behavioral dispositions). Minds are real only insofar as they can be reduced to, or shown entirely to supervene upon, matter. This is the materialism spelled out in different ways by behaviorists, identity theorists, functionalists, and (in the most extreme version, which denies the reality of the mind altogether) eliminativists.
A third alternative to Descartes is to reject both res extensa and res cogitans, and hold instead that there is some thirdkind of substance that is the only kind that really exists. What we think of as res extensa or res cogitansis really just this third kind of substance conceived of under different descriptions. One way to spell this out is the dual aspect theory of Spinoza, and another is the neutral monism of Bertrand Russell.
Then there is the view developed in recent philosophy of mind according to which there is only one kind of substance – namely material substance – but that at least some material substances have two irreducibly different kinds of attributes, namely material attributes and non-material mental attributes. On this view, materialism is right to deny a dualism of substances and to insist that material substances alone are real, but Descartes was right to insist on a dualism of attributes. This view is sometimes called attribute dualism or, more commonly, property dualism.
Now, Aquinas certainly would not agree with any of these views. He was an Aristotelian hylemorphist, and thus would reject the desiccated mathematicized conception of matter that Descartes and his successors put in place of hylemorphism. He would reject Descartes’ substance dualism as too similar to the Platonic conception of the soul’s relationship to the body, which he was keen to reject. He would reject the claims that only mind is real, or that only matter is real, or that only some third kind of thing that is neither mind nor matter is real. And he would reject the property dualist claim that only material substances are real, albeit some of them have non-material properties. (Into the bargain, he does not use the term “property” the same way that contemporary philosophers do, but that is a secondary point.)
It seems to me, however, that there is a sense in which Aquinas might arguably be classified as a property dualist, though certainly not a property dualist of the usual kind. Again, contemporary property dualists hold that only material substances exist. Aquinas disagrees with that, in part because he thinks that there are purely immaterial intellectual substances (namely angels). But I think he would also reject the suggestion that a material substance could have non-material properties. For Aquinas, the world exhibits an ontological hierarchy, from purely material things at the bottom to God at the top. Things higher up in the hierarchy can be the source of things lower down, but not vice versa. And a material substance with immaterial properties would seem to violate that principle. It would be like an inorganic substance with vegetative properties. Such a thing simply wouldn’t really be inorganic, and a material substance with immaterial properties simply wouldn’t really be a material substance.
But what about human beings, you ask? Doesn’t Aquinas think of them as material substances with immaterial properties? I’m inclined to say that that is not quite his view, or not quite what his view should be, given his broader metaphysical principles. This is where the neglected option that contemporary philosophers of mind might have seen, but seem not to, comes in. The idea of a material substance with both material and non-material properties is only one of two possible ways of spelling out property dualism. Another way of doing so would be in terms of the idea of an immaterial substance with both material and non-material properties. And that, I suggest, is essentially what Aquinas took a human being to be.
As I have argued elsewhere, most recently in an essay for the Blackwell Companion series, the Thomistic thesis that the soul is the form of the body is often misunderstood. Many people read it as saying that the soul is the form of a substance that is entirely bodily, just as the soul of a dog or a tree is the form of a substance that is entirely bodily. Then they find it puzzling that Aquinas could go on to say that the human soul subsists after death. For how could the human soul continue after death if it is the form of the body and the body is gone, any more than the soul of a dog or of a tree could subsist after death?
But this misunderstands the Thomistic thesis. The human soul is the form of a substance which has both bodily operations (like breathing, walking, seeing, etc.) and non-bodilyones (like thinking). Because it is what gives the substance in question the bodily operations in question, it is, naturally, the form of the body. But it doesn’t follow that the substance in question is entirely bodily, the way that a dog or a tree is. It is not. Even when alive, part of what we do (thinking and willing) isn’t entirely tied to the body in the first place. That’s why the death of a human being does not entail his annihilation. He carries on in a highly truncated state, reduced to his intellect and will – as an incomplete substance, as Aquinas says.
And what kind of incomplete substance is that? An immaterial one, naturally, since the body is gone. As Aquinas writes:
It must necessarily be allowed that the principle of intellectual operation which we call the soul, is a principle both incorporeal and subsistent…
[T]he intellectual principle which we call the mind or the intellect has an operation per se apart from the body. Now only that which subsists can have an operation "per se." For nothing can operate but what is actual: for which reason we do not say that heat imparts heat, but that what is hot gives heat. We must conclude, therefore, that the human soul, which is called the intellect or the mind, is something incorporeal and subsistent.
End quote. Now, a human being is one substance, not two. Again, Aquinas rejects the Platonic view of the soul and would reject the Cartesian substance dualist view as too similar to it. But a human being is not an angel, because angels have no bodily properties or operations at all and human beings do. The implication seems clearly to be that a human being is an immaterial substance that has material or bodily operations and properties as well as immaterial ones. In which case Aquinas is a property dualist of a sort. And notice here that “property” in Aquinas’s sense of that term (and not just in the contemporary analytic philosopher’s sense of the term) is indeed the right word, because our bodily activities (again, breathing, walking, seeing, etc.) are indeed proper to us. They are proper accidents of a human being rather than merely contingent ones. A human being in his mature and healthy state will exhibit these bodily properties, which is why death is for Aquinas not a liberation (as it is for Plato). It is, as I have put it elsewhere, something like a “full body amputation” – the loss of all of the bodily properties that a complete and fully functioning specimen of our kind would exhibit, leaving only the non-bodily properties.
Again, I say this tentatively. Aquinas was not addressing precisely the issues contemporary philosophers are, and he does not use the relevant terminology in exactly the same way. So a claim like “Aquinas was a kind of property dualist” is bound to be easily misunderstood. It is bound to raise in some people’s minds connotations that I do not intend and that Aquinas would not accept. But it seems to me that, suitably qualified, there is a sense in which it is true.
Further reading: