Does matter exist? Bishop Berkeley famously said, No, a thesis which has earned him a reputation as a brilliant crank (emphasis on “crank”). Here’s his argument:
Wood, stones, fire, water, flesh, iron, and the like things, which I name and discourse of, are things that I know. And I should not have known them but that I perceived them by my senses; and things perceived by the senses are immediately perceived; and things immediately perceived are ideas; and ideas cannot exist without the mind; their existence therefore consists in being perceived. (Dialogues of Hylas and Philonous, 3.20)
Common sense tells us that we perceive things external to ourselves, and it equally tells us that we do so by way of the senses. Things get spicier with his second and third premises, about immediate perception: first, that whatever we perceive by the senses, we perceive immediately; and second, that whatever we perceive immediately is an idea. Descartes and Locke both endorse something like each of these premises, but as separate operations: so, the immediate action of external bodies on our senses is thought to yield a mental Idea (Locke’s term), which we perceive immediately. We’re to imagine ourselves as locked inside our skulls, as it were, with only mediated access to the external world, a mediation achieved by the resemblances between Ideas and things. Berkeley insists that this view — mental representationalism — leads quite naturally to skepticism, as Hume was soon to prove. His innovation within this philosophical tradition is to conflate the two operations, by eliminating the “external bodies”: we perceive things as external; but since we can only perceive ideas, those external things must simply be ideas, existing as perceived and caused by another mind, namely, God’s.
We’re apt, with Hylas (from the Greek hyle, “matter”) to recoil with dismay from this statement; it sounds as though it reduces the external world to a mere fiction. But Berkeley is adamant, in the quotation above and throughout the Dialogues, that he begins and ends with the sensible, external world — the danger of belief in matter is that it tempts us to drive a wedge between our sensory experience and the material substrate that occasions it (cf. esp. Locke’s distinction of primary and secondary qualities (roughly, extension and solidity on the one hand, and everything else — color, texture, sound, etc. — on the other), with the latter having no essential connection to things themselves, whatsoever). It’s Locke and not Berkeley who leaves us wondering what a cherry really is, if not something red, squishy, and sweet.
The great question he faces, though, is whether the “idea-immediacy premise” is true: do we actually only perceive our ideas? Aquinas raises this question quite directly, in ST I.85.2, where he considers whether the “intelligible species” of a percipiendum is what the intellect understands (with Locke, or Berkeley), or that “by which” it understands. He notes, “Some have asserted that our intellectual faculties know only the impression made on them,” and makes it clear below that he is thinking at least of the sophist Protagoras, for he notes that this opinion would yield the result that “whatever seems, is true…Thus every opinion would be equally true; in fact, every sort of apprehension” (I.85.2.c.).
This is an objection that Berkeley is keenly aware of, and he devotes a great deal of work to avoiding the sophistic conclusion that idealism means that “man is the measure of all things.” So, Hylas asks how, if there are only ideas and not material things, we can call a man mistaken who, perceiving that an oar placed in water looks crooked, judges that it is so (3.52) To this, Philonous maintains two things: first, this man “is not mistaken with regard to the ideas he actually perceives [i.e., the oar genuinely does look crooked under the given circumstances], but in the inference he makes from his present perceptions. Thus, in the case of the oar, what he immediately perceives by sight is certainly crooked; and so far he is in the right. But if he thence conclude that upon taking the oar out of the water he shall perceive the same crookedness; or that it would affect his touch as crooked things are wont to do: in that he is mistaken” (3.56). As Philonous put it earlier, imagined ideas are 1) “faint and indistinct,” 2) dependent on the will, and 3) “not…connected, and of a piece with the preceding and subsequent transactions of our lives” (3.44). The third claim is most important for our purposes here: false or fabricated ideas don’t have a place in an ideal scientific description of the universe, which God in his wisdom and goodness toward us, has created to operate in a regular, mathematically describable fashion.
So, it’s not initially clear that Aquinas’s concerns about the relativistic or skeptical entailments of the idea-immediacy premise apply to Berkeley; at the very least, he’s quite as keen to avoid those conclusions as Aquinas is. In any case, here is the realism with which Aquinas opposes Protagorean relativism: “The intelligible species is related to the intellect as that by which it understands…The intelligible species is that which is understood secondarily; but that which is primarily understood is the object, of which the species is the likeness” (ST I.85.2.c.). As John O’Callaghan insists, it never occurs to Thomas to question whether we know “corporeal things.” Instead, he considers what we know them through, in what manner we know them, and what in them we know (Thomist Realism and the Linguistic Turn, 199).
What does this amount to? Aquinas shares Berkeley’s conviction that we have real knowledge of an external world — they mutually loathe any epistemology, such as Locke’s, which would interpose mental entities between mind and world. And Aquinas also shares Berkeley’s (and Locke’s) conviction that the mind cannot immediately know a material thing (a chair cannot lodge itself inside my mind); it has to know that material thing in an intellectual way, as an “intelligible species.”
Where they really disagree, it seems to me, is over the coherence of positing some kind of causal connection between spirit and matter. Berkeley cannot comprehend the notion of matter acting on spirit, or vice versa. Aquinas, however, insists, “It is not correct to say that as the sense knows only bodies so the intellect knows only spiritual things; for it follows that God and the angels would not know corporeal things. The reason of this diversity is that the lower power does not extend to those things that belong to the higher power; whereas the higher power operates in a more excellent manner those things which belong to the lower power” (ST I.84.1.ad 2). Matter does not act on spirit, but spirit can act on matter — Aquinas takes it that both are simply compositions of potency and act, that is, of being and non-being, with spirit being relatively more actualized than matter; the two aren’t qualitatively different substances, as with Descartes, but rather variously — i.e., analogously — concentrated instances of being itself, cf. ST I.3.2).
Pure matter, then, is something of a placeholder for Aquinas, an unattainable horizon of pure potency, of utter formlessness. It’s not, as it is for Locke, a distinct, unknowable substrate underlying things as they appear; rather, it’s something we posit as that which the various categorical determinations of being inform (substance, quantity, quality, relation, etc.). Aristotle sums up this view of matter nicely: “It would seem that the form and the combination of form and matter are more truly substance than matter is. The substance, then, which consists of both—I mean of matter and form—may be dismissed [i.e., set aside for now], since it is posterior and obvious. Matter too is in a sense evident” (Metaphysics, 1029a). The existence of individual things, Aristotle takes it, is obvious; similarly obvious, he observes, is the existence of the matter which they inform. (The problematic entities for Aristotle, given their legacy in Plato’s thought, are forms.)
But is the existence of matter so obvious, or so necessary, even in an Aristotelian scheme? Consider another quotation from Aristotle: “We do cause a bronze sphere to be, for we produce it from bronze and a sphere; we induce the form into this particular matter, and the result is a bronze sphere” (Ibid., 1033b). What work is the word “matter” doing in that sentence? Couldn’t we substitute “lump of bronze” for it, without loss of sense or truth? I can hear Berkeley gently urging Aristotle and Aquinas, “Friends, you have no need of that hypothesis!”