Here as in other areas of philosophy, misunderstanding arises because contemporary readers are usually unaware that classical (Aristotelian/Neo-Platonic/Scholastic) philosophers and modern (post-Cartesian) philosophers carve up the conceptual territory in radically different ways, and thus often don’t use key terms in the same sense. In this case, terms like “matter” and “material” have a very different force when writers like Aristotle and Aquinas use them than they have when Descartes, Hobbes, or your average contemporary academic philosopher uses them. There are at least three ways in which this is true.
The matter of the moderns
First, and as I have noted many times, the tendency in post-Cartesian philosophy and natural science is to conceive of matter in exclusively quantitative terms and to regard whatever smacks as irreducibly qualitative as a mere projection of the mind. This is the origin of “the qualia problem” for materialism. The reason materialists cannot solve the problem is that since they have defined matter in such a way as to exclude the qualitative from it, qualia -- which are essentially qualitative, as the name implies -- are necessarly going to count as immaterial. Materialist “explanations” of qualia thus invariably either change the subject or implicitly deny the existence of what they are supposed to be explaining. (The basic point goes back to Cudworth and Malebranche and is the core of Nagel’s critique of physicalist accounts of consciousness.)
This is a point I‘ve developed at length many times (e.g. here, here, here, here, here, and here) and I won’t belabor it here. Suffice it to say that for the A-T philosopher, while this is a strike against materialism it isn’t really an argument for dualism unless one accepts the purely quantitative conception of matter in question -- as Cartesians do but A-T does not. From an A-T point of view, the modern “mathematicized” conception of matter is essentially incomplete. It’s true as far as it goes, but it’s not the whole truth. So, the failure of some feature to be analyzable in material terms as materialists and Cartesians understand “material” does not entail that it is not material full stop. It might still count as material on some more robust conception of matter. And there is a sense in which, for A-T, qualia are indeed material, at least if we use “material” as more or less synonymous with “corporeal.” For A-T philosophers regard qualia as entirely dependent on physiology. Our having the qualia associated with seeing a red object, for example, is entirely dependent on bodily organs like the retina, the optic nerve, the relevant processing centers in the brain, and so forth.
This brings us to the second way in which A-T philosophers carve up the conceptual territory in ways contrary to the assumptions typically made by modern philosophers. For some modern dualists are bound to object: How, on any conception of matter, could qualia be entirely dependent on such bodily organs? Don’t attempts to analyze qualia in terms of (say) neuronal firing patterns fail whether or not we think of matter as exhaustively quantitative? The trouble with such objections, though, is that they think of materiality or corporeality in essentially reductionist terms. They suppose that to say that such-and-such a feature is corporeal entails saying that it is reducible to some lower-level feature of the body. Hence when they hear the A-T philosopher say that qualia are corporeal and dependent on bodily organs like the brain, they suppose that the A-T philosopher is claiming (as a materialist might) that an experience of red is “nothing but” the firing of such-and-such neurons, that an experience of pain is “nothing but” the firing of some other group of neurons, etc.
But that is simply a fundamental misunderstanding of the A-T position. The A-T philosopher entirely rejects the reductionist assumption that lower-level features of a system are somehow “more real” than the higher-level features, or in any other way metaphysically privileged. Hence he rejects the idea that to affirm that some feature of the world is both real and material is to suppose that it is exhaustively analyzable into, or entirely reducible to or emergent from, some collection of lower-level material features. (The words “exhaustively” and “entirely” are crucial here. Naturally, the A-T philosopher does not deny that a system can be analyzed into its parts and that this has explanatory value. The point is that this is only part of the story. The parts in turn cannot properly be understood except in relation to the whole, at least in a true substance as opposed to an artifact. See chapter 3 of Scholastic Metaphysics for detailed treatment of this issue, including responses to the usual objections.)
Within the material world, A-T philosophers traditionally hold that there are at least four irreducible kinds of substance: inorganic substances; merely vegetative organic substances (in the technical Aristotelian sense of “vegetative”); sensory or animal substances; and rational animals or human beings. Only in the case of the last does the A-T position hold that there is a strictly immaterial or incorporeal aspect. Non-human animal life is irreducible to vegetative life and vegetative life is irreducible to the inorganic, yet all are still entirely material. Again, materiality or corporeality simply has nothing essentially to do with reducibility.
So, in order to understand what A-T philosophers mean by “matter” and “material,” the reader must be careful not to read into their statements the exclusively quantitative construal of “matter” or the reductionist construal of “material” that are at least implicit in the usage of the average modern philosopher. How, then, does the A-T philosopher understand “matter” and “material”?
Degrees of immateriality
This brings us to the third point, which is that from the A-T point of view, matter is to be understood primarily in contrast to form, where the matter/form distinction is a special case of the more general distinction between potentiality and actuality. Consider a triangle drawn on a whiteboard with a dry-erase marker. It is a composite of a certain form, triangularity, and a certain kind of matter, ink. (Metaphysically, things are more complicated than that, since the triangle is an artifact and thus triangularity is an accidental form modifying something already having a substantial form; and the ink, accordingly, is a kind of secondary matter, rather than the prime matter that substantial forms inform. But we can ignore all that for present purposes. Again, see Scholastic Metaphysicsfor the full story.)
The ink qua ink is potentially a triangle, or a circle, or a square, or some other figure. The form triangularitymakes it actually one of these rather than the others. The form triangularityis of itself universal and one. That is to say, it is the same one form -- triangularity -- that is instantiated in this triangle, in other triangles drawn on the whiteboard, in triangles drawn in geometry textbooks or in sand at the beach, etc. By contrast, the specific bit of ink that has taken on that form on the whiteboard is particular, and makes of the triangle a mere particular instance of triangularity among multiple particular instances. That it is made of this particular bit of ink also makes the triangle changeableand imperfect. The triangle can be damaged or erased altogether, and even when it exists it does not instantiate triangularity perfectly, insofar as the sides of any material triangle are never perfectly straight, etc. By contrast, triangularity as such is perfect triangularity, and indeed is the standard by reference to which particular instances of triangularity are judged more or less perfect or imperfect. Triangularity as such is also permanent. Individual triangles change and are generated and corrupted, but triangularity as such is timeless and unchanging.
So, form qua form corresponds in A-T metaphysics to actuality, universality, unity, permanence, and perfection. Matter qua matter corresponds to potentiality, particularity, multiplicity, changeability, and imperfection. Now, these characteristics are susceptible of degrees, so that there is a sense in which materiality and immateriality can come in degrees. The more something exhibits potentiality, particularity, multiplicity, changeability, and/or imperfection, the more matter-like it is. The more something exhibits actuality, universality, unity, permanence, and/or perfection, the more immaterial it is. It is in light of this that we can understand how, though A-T regards perceptual experience (and the qualia associated with it) as corporeal, there is nevertheless a sensein which it has an immaterial aspect.
For A-T epistemology, knowledge or cognition involves a kind of union of the knower and the thing known insofar as the former comes, in a sense, to possess the form of the latter. Now, knowledge or cognition can be either of a sensory sort or of an intellectual sort. The first sort we share with other animals; the second is the sort we have and other animals do not. It is the second, intellectual sort of cognition that is in the strict sense immaterial and is thus incorporeal. But sensory cognition, though corporeal, is immaterial in a loose sense insofar as there is a way in which it involves having the form of the thing known without having its matter.
Consider the perceptual representation of an apple that you form when you look at it. The color, part of the shape, and the appearance of the texture of the apple are captured in the visual experience, whereas the interior of the apple, its weight, its solidity, and other characteristics are not captured. By capturing the former without the latter, the visual experience involves a kind “dematerialization,” as it were. It “pulls” the forms redness, roundness, etc. from the apple so that they exist as qualia of conscious experience rather than in the apple itself, while “leaving behind” the rest of the apple. But this is not a strict dematerialization, of course, any more than is the “dematerialization” accomplished by a photorealistic still life painting of the apple (which also captures the color, shape, etc. without capturing the interior of the apple, its weight and solidity, etc.). For just as the painting is itself embodied in canvas and paint, which are material, so too is the perceptual experience embodied in physiological activity, which is also material.
Now, the loose sort of “dematerialization” accomplished by physiological activity can be more thoroughgoing than the sort involved in a perceptual experience. The visual experience of the apple is an experience of this particular apple, capturing its particular color, shape, etc. But a mental image of an apple might resemble many apples -- say, by virtue of more vaguely capturing the color or shape, or by leaving out features such as idiosyncratic indentations or areas of discoloration. And other representations encoded physiologically (such as those posited by cognitive scientists) might be even further than a vague visual image is from physically resembling any particular thing, as a blueprint or wiring diagram is very far from resembling any actual building or computer. This distance from the kind of close resemblance between a representation and particular thing represented that is involved in a perceptual experience gives mental images and more abstract neural representational states a kind of generality which can superficially resemble the universality of concepts. This distance from the particular things thus makes these representations “immaterial” in a loose sense.
Still, strictly speaking, they are material. And neither neural representations nor anything else material can in principle have the true universality of reference that concepts have, nor the determinate or unambiguous content that concepts can have. For material representations will of their nature have particularizing features that prevent them from capturing the universality of a concept, and will be systematically indeterminate or ambiguous between alternative possible semantic properties. Hence, just as you will never get a true circle from a polygon no matter how many sides you add to it, you will never get a true concept from a material representation, no matter how many particularizing features are removed from it, and no matter how many other representations you add to it in a system of material representations in order to narrow down the range of possible semantic contents. In both cases, you can at best only get a simulation. To be sure, the simulation might be very impressive. A polygon with sufficiently many sides can fool the eye and appear to be a circle. A sufficiently powerful computer program might appear to be intelligent. But if you examine any polygon carefully enough its non-circularity is bound to become evident, and if you examine the outputs of any computer carefully enough its “sphexishness”is bound to become evident.
The thesis that concepts are in principle irreducible to material representations is something I’ve defended at length elsewhere, most systematically and in greatest depth in my ACPQ article “Kripke, Ross, and the Immaterial Aspects of Thought.” (Some relevant blog posts can be found here, here, here, here, and here.) Anyway, arguing for the immateriality of thought is not the point of the present post. The point is to note that on the A-T view, whereas sensation and imagination are immaterial in a loose sense, conceptual thought is immaterial in a strict sense.
Even then there is the qualification to be made that the human intellect must constantly “turn to the phantasms,” as Aquinas puts it -- that is, it depends on sensation for the raw materials from which it abstracts concepts, and it makes use of mental imagery even when entertaining the most abstract concepts. For instance, the concept triangularitycannot be identified with any mental image of a triangle nor with the word “triangle,” but we tend to form images either of the geometrical figure or of the word whenever we entertain the concept. (Previous posts with some relevant discussion can be found here, here, here, and here.) As rational animals we are composites of the corporeal and incorporeal and are thus not entirely divorced from matter even in our intellectual activity. Only an essentially incorporeal intellectual substance -- an angel, or God -- would be that.
Hence we find in A-T writers a distinction between three degrees of immateriality:
1. The quasi-immateriality or “immateriality” in a loose sense of sensations, mental images, and other neural representations. These we share with the lower animals. The “immateriality” is loose because these are all corporeal or intrinsically dependent on matter.
2. The strict immateriality of true concepts. These we do not share with the lower animals. But, though not intrinsically dependent on matter, our intellectual or conceptual activity is extrinsically dependent on matter insofar as we require sensation and mental imagery -- and thus sense organs and brain activity -- as a source of information and as an accompaniment to the act of thinking.
3. The absolute independence of matter of angelic intellects and the divine intellect, which do not require bodily organs even extrinsically.