Classical arguments for the immateriality of the mind, by which I mean the sort common within Western philosophy prior to Descartes and defended by the likes of Plato, Aristotle, and Aquinas, are very different. You won’t find the latter thinkers going on about either qualia or intentionality, because the very notions of qualia and intentionality, as usually understood, are artifacts of the modern mechanistic re-conception of the material world. “Qualia” are what you get when you deny that matter can have anything like the sensible qualities it seems to have in ordinary experience. “Intentionality” is what you get when you insist that the material world is devoid of anything like final causality, when you go on accordingly to relocate all meaning and purpose within the mind, and when you also go on in turn to characterize mental states as internal “representations” of an external reality. I have said a little bit about all of this in earlier posts, and it is a theme I explore in great detail in The Last Superstition.
For Plato, Aristotle, Aquinas, and other ancients and medievals, the main reason why the mind has to be immaterial concerns its affinity to its primary objects of knowledge, namely universals, which are themselves immaterial. When properly fleshed out and understood, this sort of argument is in my view decisive. Yet it has received very little attention from contemporary philosophers, partly, I think, because of their general ignorance of what the ancients and medievals thought, and partly because the logic of the mechanistic revolution inaugurated by Galileo, Descartes, Hobbes, Locke, et al. has pushed them into so cramped and narrow a conceptual space that they can hardly even conceive any alternative to it. The result is that when they do address the arguments of the ancients and medievals (concerning this subject or any other), they almost always distort them in the most grotesque fashion, anachronistically reading into them assumptions that make sense only if one takes for granted conceptions of matter, mind, causation, etc. that the older thinkers in question would have regarded as deeply mistaken and muddleheaded. (Thus is Aristotle made out to be a “functionalist” vis-à-vis the mind, Aquinas’s Fifth Way is read as if it were an anticipation of Paley’s feeble “design argument,” etc.)
In The Last Superstition, I explain at length why some form of realism about universals is rationally unavoidable. (Whether it is the Platonic form of realism, the Aristotelian one, or the Scholastic one that we should endorse is a separate matter irrelevant to present purposes.) I am not going to attempt to summarize that case here, but the examples to follow should suffice to give a sense of how an argument from the reality of universals to the immateriality of the mind might proceed. Readers wanting a fuller treatment should consult TLS.
Consider that when you think about triangularity, as you might when proving a geometrical theorem, it is necessarily perfect triangularity that you are contemplating, not some mere approximation of it. Triangularity as your intellect grasps it is entirely determinate or exact; for example, what you grasp is the notion of a closed plane figure with three perfectly straight sides, rather than that of something which may or may not have straight sides or which may or may not be closed. Of course, your mental image of a triangle might not be exact, but rather indeterminate and fuzzy. But to grasp something with the intellect is not the same as to form a mental image of it. For any mental image of a triangle is necessarily going to be of an isosceles triangle specifically, or of a scalene one, or an equilateral one; but the concept of triangularity that your intellect grasps applies to all triangles alike. Any mental image of a triangle is going to have certain features, such as a particular color, that are no part of the concept of triangularity in general. A mental image is something private and subjective, while the concept of triangularity is objective and grasped by many minds at once. And so forth. In general, to grasp a concept is simply not the same thing as having a mental image. (Again, see TLS for more details.)
Now the thought you are having about triangularity when you grasp it must be as determinate or exact as triangularity itself, otherwise it just wouldn’t be a thought about triangularity in the first place, but only a thought about some approximation of triangularity. Yet material things are never determinate or exact in this way. Any material triangle, for example, is always only ever an approximation of perfect triangularity (since it is bound to have sides that are less than perfectly straight, etc., even if this is undetectable to the naked eye). And in general, material symbols and representations are inherently always to some extent vague, ambiguous, or otherwise inexact, susceptible of various alternative interpretations. It follows, then, that any thought you might have about triangularity is not something material; in particular, it is not some process occurring in the brain. And what goes for triangularity goes for any thought that involves the grasp of a universal, since universals in general (or at least very many of them, in case someone should wish to dispute this) are determinate and exact in a way material objects and processes cannot be.
As James F. Ross has argued, some of the best-known arguments of twentieth-century analytic philosophy reinforce this judgment. For instance, Quine’s arguments for the indeterminacy of translation and Kripke’s argument regarding “quaddition” show that there is in principle nothing in the facts about human behavior or physiology, or in any other physicalistically “respectable” set of facts, that can determine (say) whether by “gavagai” I mean “rabbit” or “undetached rabbit part,” or whether I am doing addition rather than “quaddition.” Indeed, these arguments show that this same indeterminacy afflicts everything I say or do. Yet it is simply false that everything I say or do is indeterminate in this way. For example, should I deploy modus ponens in defending a Quine- or Kripke-style argument, what I will be deploying is indeed modus ponens and not some mere approximation of modus ponens; certainly it had better be modus ponens and not some mere approximation, otherwise my arguments would all be invalid. Nor will it do to suggest that maybe all my arguments really are invalid, for even to deny that I ever really use modus ponens but only ever approximate it requires that I first grasp determinately what modus ponens is before judging that I never really engage in it. Similarly, if someone wanted to deny that we ever really grasp perfect triangularity, he would first have to grasp it himself before going on to judge (obviously falsely, in that case) that it is something we never grasp.
So, there is no coherent sense to be made of the suggestion that all of our thoughts are indeterminate. But if at least some of them are determinate, and no physical process or set of physical facts is ever determinate, it follows that at least some of our thoughts are not physical. (Ross’s argument, by the way, is elegantly developed in his article “Immaterial Aspects of Thought,” which appeared in the Journal of Philosophy in 1992. A later version of this article is available at his website, in the form of a chapter of his book manuscript Hidden Necessities.)
That is one way an argument from realism about universals to the immateriality of the mind can be developed. There are other ways too, which I will summarize in future posts.
Whatever one thinks of arguments like this, it is important to understand that (like the other arguments I’ve presented in this series) they are not the sort that might be undermined by the findings of neuroscience, or any other empirical science for that matter. They are not “soul of the gaps” arguments which purport to give a quasi-scientific explanation of some psychological phenomenon that we simply haven’t got enough empirical data to explain in a materialistic way. Rather, they purport to show that it is in principle impossible, conceptually impossible, for the intellect to be accounted for in a materialistic way. If such arguments work at all, they establish conclusively that the intellect could no more be identified with processes in the brain than two and two could make five. If they are mistaken, they would be mistaken in the way one might make a mistake in attempting to carry out a geometrical proof, and not by virtue of having failed to take account of this or that finding of brain research.