Everyday experience tells us that a human being is the sort of thing that eats, sleeps, grows, reproduces, sees, hears, walks, feels, loves, hates, speaks, thinks, and chooses. Aristotle’s way of summing up this homely truth was to say that we are by nature rational animals. That we are animals is thus something we hardly needed Darwin to tell us. It is obvious from the fact that, like other animals, we have stomachs and skin, eyeballs and ears, limbs and teeth, muscles, brains, and the other organs necessary to carry out the activities in question. Like dogs and cats, apes and eels, we are essentially bodily creatures.
Yet it doesn’t follow that we are mere animals, and our rationality is what sets us apart from the rest of the genus. Indeed, for Aristotle, and for Aquinas after him, rationality is unlike our other capacities in having an essentially immaterial and non-bodily aspect. The reason has to do with our capacity to form abstract concepts, which underlies all our other distinctively rational activities. It is because you can grasp what it is to be a man -- not just this particular man or that one, but any possible man, man as a universal -- that you can go on to form judgments like the judgment that all men are mortal, can reason from that judgment together with the judgment that Socrates is a man to the conclusion that Socrates is mortal, and so forth.
There are several arguments that establish that this capacity for abstract thought cannot in principle be reduced to or otherwise entirely explained in terms of brain activity, even if brain activity is part of the story. The arguments have their roots in Plato and Aristotle and have been defended in recent years by Aristotelian philosophers like Mortimer Adler, John Haldane, David Oderberg, and James Ross.1 Answering the various objections to (and misunderstandings of) these arguments takes some work, but the basic idea can be set out fairly simply.2
Let us take as an example the thought that triangles have three sides. For that thought (or any other) plausibly to be material, it would have to be identifiable with something like a symbol or set of symbols encoded in the brain -- something analogous to the symbols encoded in the electronic circuitry of a computer. But there is no way a thought could be entirely reducible to that sort of thing. For no material symbol could possibly have the determinate or unambiguous content that at least many of our concepts have; and no material symbol could possibly have the universal reference that our concepts have.
Consider the most unambiguous symbol of triangularity there could be -- a picture of a triangle, such as the one to the right. Now, does this picture represent triangles in general? Or only isosceles triangles? Or only small isosceles triangles drawn in black ink? Or does it really even represent triangles in the first place? Why not take it instead to represent a dinner bell, or an arrowhead? There is nothing in the picture itself that can possibly tell you. Nor would any other picture be any better. Any picture would be susceptible of various interpretations, and so too would anything you might add to the picture in order to explain what the original picture was supposed to represent. In particular, there is nothing in the picture in question or in any other picture that entails any determinate, unambiguous content. And even in the best case there is nothing that could make it a representation of triangles in general as opposed to a representation merely of small, black, isosceles triangles specifically. For the picture, like all pictures, has certain particularizing features -- a specific size and location, black lines as opposed to blue or green ones, an isosceles as opposed to scalene or equilateral shape -- that other things do not have.
Now what is true of this “best case” sort of symbol is even more true of linguistic symbols. There is nothing in the word “triangle” that determines that it refers to all triangles or to any triangles at all. Its meaning is entirely conventional; that that particular set of shapes (or the sounds we associate with them) have the significance they do is an accident of the history of the English language. But something similar could be said of any material symbols whatever. Even if we regarded them as somehow having a built-in meaning or content, they would not have the universality or determinate content of our concepts, any more than the physical marks making up the word “triangle” or a picture of a triangle do. But then the having of a concept cannot merely be a matter of having a certain material symbol encoded in the brain, even if that is part of what it involves. Nor can it merely be a matter of having a set of material symbols, or a set of material symbols together with certain causal relations to objects and events in the world beyond the brain. For just as with any picture or set of pictures, any set of material elements will be susceptible in principle of alternative interpretations; while at least in many case, our thoughts are not indeterminate in this way.
We might understand the point by analogy with sentences. If you are going to use the English sentence “Snow is white,” you are typically going to have to express it via some material medium -- ink marks, pixels, sound waves, or what have you. All the same, the meaning of that sentence cannot be accounted for in terms of any of the physical properties of those media. There is nothing in the shapes of the letters that make up the words of the sentence, or the chemistry of the ink in which they are written, or the physics of the compression waves in the air that you generate when uttering them, that makes them refer to snow or to whiteness or indeed to anything at all. A sentence is a seamless unity of the material and the immaterial, and it is created by another seamless unity of the material and immaterial -- a human being.
At this point there will no doubt be those who object that positing ectoplasm or spook stuff is hardly a better explanation of thought than an appeal to brain activity is. And that is quite true. But then, I said nothing about ectoplasm or spook stuff in the first place. When a mathematician points out that it is just muddleheaded to speak of the square root of 25 as if it were a kind of physical object, it would be silly to accuse him of believing that the square root of 25 is made out of ectoplasm or spook stuff. If your picture of reality cannot accommodate numbers alongside physical objects, that is your problem, not his. Mathematics simply provides a powerful example of a body of truths that cannot be captured in the language of physics, chemistry, neuroscience, and the like.
Similarly, to point out that whatever a thought is, it cannot in principle be reduced to the physical properties of brain activity, is simply to provide another example of an aspect of reality that cannot be entirely captured in such language. Only if we assume that all of reality must be so captured will this sound odd, but that we should not assume this is, of course, precisely the point. And if we do assume it, we are doing so in the face of the evidence, and not on the basis of the evidence. For it is precisely what we know about thought from our everyday familiarity with it -- such as the fact that it sometimes has a determinate content, and a universal reference -- that tells us that it cannot be entirely material, just as it is what we know about numbers from our everyday familiarity with them that tells us that they cannot be physical objects.
But doesn’t neuroscience show that there is a tight correlation between our thoughts and brain activity? It does indeed. So what? If you smudge the ink you’ve used to write out a sentence or muffle the sounds you make when you speak it, it may be difficult or impossible for the reader or listener to grasp its meaning. It does not follow that the meaning is reducible to the physical or chemical properties of the sentence. Similarly, the fact that brain damage will seriously impair a person’s capacity for thought does not entail that his thoughts are entirely explicable in terms of brain activity.
Aristotle and Aquinas, though they regarded the human intellect as immaterial, would not have been surprised in the least by the findings of modern neuroscience. Indeed, they would have been surprised had neuroscience not turned up the correlations it has. This will sound surprising if you take Descartes as your paradigm of a philosopher who affirms the immateriality of the human mind. But defending Descartes is exactly the reverse of what I have been doing. For it was Descartes who substituted the real, concrete human being -- a seamless unity of the physical and the mental, the bodily and the immaterial -- with a bizarre patchwork of abstractions of his own devising. Materialists have followed him ever since. Materialism is just a riff on Cartesianism, not its opposite. Tomorrow, I’ll explain exactly what I mean.
1. See Mortimer Adler, Intellect: Mind Over Matter (New York: Collier Books, 1990); J. J. C. Smart and J. J. Haldane, Atheism and Theism, Second edition (Oxford: Blackwell, 2003), pp. 96-109; David S. Oderberg, “Hylemorphic Dualism,” Social Philosophy and Policy 22 (2005); and James Ross, “Immaterial Aspects of Thought,” Journal of Philosophy 89 (1992).
2. I provide an exposition and defense of such arguments in chapter 7 of my book Philosophy of Mind and chapter 4 of my book Aquinas. An especially detailed exposition and defense can be found in my article “Kripke, Ross, and the Immaterial Aspects of Thought,” forthcoming in the American Catholic Philosophical Quarterly.