I ended part one by saying that 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 between brain activity on the one hand, and on the other hand our distinctive capacity for abstract, conceptual thought. They affirmed that, much as a sentence is a seamless unity of the immaterial and the material -- of meaning and of the physical medium by which meaning is conveyed -- so too a human being is another seamless unity of the immaterial and the material. Finally, I suggested that it was actually Descartes who substituted the real, concrete human being with a bizarre patchwork of abstractions of his own devising, and set the stage for the various materialisms that followed. It is that point to which I will return today.
Descartes was among those early modern thinkers -- Galileo and Newton were two others -- who put at the center of Western thought an essentially mathematical conception of the material world. There is nothing wrong with that conception when it is used for the purposes for which it was invented -- the prediction and control of natural phenomena. But it is merely an abstraction from concrete material reality, and not the whole of material reality. It captures only those aspects that are susceptible both of mathematical modeling and of detection via the experimental techniques by which the models may be tested. But anything else falls through its net.
Now when aircraft engineers abstract from real, concrete passengers everything but their average weight, so as to determine how many passengers of average weight can fly on a particular type of plane, no one thinks that the utility of this method shows that actual airline passengers have no features other than their average weight. The abstraction is extremely useful, but hardly provides an exhaustive description of reality. Yet the utility and predictive power of physics is often taken as evidence that it gives us an exhaustive picture of material reality. This sort of argument is like that of the drunk who insists that his keys have to be under the streetlamp, because that is the only place there is light to look for them. That it is difficult to look for the keys elsewhere has absolutely no tendency to show that they are not in fact elsewhere. And that it is difficult to study aspects of material reality not susceptible of the methods of physics does not show that no such aspects exist. The fallacy is extremely obvious when you think about it, but it is committed all the time by otherwise intelligent people, and gains a certain prestige from the frequency of its commission and the status of those who commit it.
Now Descartes not only took the material world in general to be exhausted by what could be put in the abstract language of mathematics, but he took the human body in particular to be so exhausted. Colors, sounds, smells, tastes, heat, and cold, as common sense understands these qualities, are, precisely because they are qualitative rather than quantitative, not expressible in the mathematics that has become the language of physics. Descartes thus relegated them to the mind. Color, sound, heat, cold, etc., as common sense understands them, anyway, were in his view to be regarded as existing only in our experience of the material world, not in the material world itself.
But neither, of course, are notions like meaning, thinking, willing, or other mental attributes expressible in the language of physics. Hence, having already reified one abstraction -- that is to say, having taken the abstracted mathematical features of the concrete material world and treated them as if they just were the whole of the material world -- Descartes went on to reify another. He abstracted conscious thought from the concrete human being who is its subject, and made of it a substance in its own right, a res cogitans or “thing that thinks.”
A human being was then redefined by Descartes as a composite of these two invented substances: “matter,” understood not as common sense understands matter but rather as an abstract mathematical structure treated as if it were a concrete reality, and utterly devoid of color, odor, sound, taste, heat, cold, meaning, or purpose; and “mind,” understood not as a faculty of a certain kind of animal but rather as an entity in its own right, a thinking thing that is nothing but a thinking thing, and in which alone exist the qualitative features we know from conscious experience (“qualia,” as contemporary philosophers call them).
Naturally, this bizarre re-conception of human nature -- man as a “ghost in a machine,” as Gilbert Ryle famously parodied it -- opened up a host of philosophical problems which persist to this day. The materialist “solution” to the problems has been to reject one of Descartes’ reified abstractions (the res cogitans) while keeping the other, the material world conceived as if the equations of physics exhausted its nature. Unsurprisingly, this has led to theories of the relationship of mind to body which seem implicitly to deny, rather than to explain, the existence of mind, consciousness, meaning, and free choice. And as books like Alex Rosenberg’s recent The Atheist’s Guide to Reality indicate, it seems that an increasing number of materialists might be willing to make the denial explicit. But the view that results is utterly incoherent -- in effect, a denial of the very existence of rational thought in the name of rational thought.
A genuine solution requires abandoning both of Descartes’ abstractions and rediscovering the human being as an irreducible psychophysical whole -- the mental and the physical as two aspects of one thing, just as a sentence’s meaning and the physical marks that make it up are two aspects of one thing. Far from being “unscientific,” such a rediscovery would be a return to reality as we actually know it from experience -- rather than the fallacious and ideological retreat from experience represented by Cartesianism and materialism alike. Wittgenstein and his followers made a significant contribution to such a rediscovery. (M. R. Bennett and P. M. S. Hacker’s Philosophical Foundations of Neuroscience is an important recent example.) But it is not enough to critique bad metaphysics (as Wittgensteinians essentially confine themselves to doing). A sound metaphysics -- a metaphysics that builds on rather than denies what we know through ordinary experience about human beings and the wider natural world -- must be put in its place. And that is what Aristotle, Aquinas, and their contemporary successors offer.