A key step in Hume’s argument against the rationality of induction is the following: “It is agreed on all hands that there is no known connection between the sensible qualities and the secret powers,” “on which the influence of these objects entirely depends” (Inquiry, 4.2). This seems to me effectively to be a more radical version of Locke’s distinction between primary and secondary qualities. Locke argued, “Primary qualities are whatever is inseparable from the body itself (solidity, figure, extension, etc.) (2.8.9). Secondary qualities “are nothing in the objects themselves, but powers to produce various sensations in us by their primary qualities”: “colors, sounds, tastes” (Essay, 2.8.10). Locke takes it that we more or less know primary qualities immediately — the very idea of “body” includes in its concept the idea of extension and solidity — but that secondary qualities (color, texture, temperature, taste, etc.) have an indeterminate relationship to the thing itself (2.8.12-13). For Hume, even this won’t really do — every quality we associate with things in the world is tied to that thing by custom and experience, and that’s all (cf. Inquiry, 5.2). (There’s almost certainly a lineal connection to be drawn from Locke and Hume to Kant’s distinction between the noumenal and phenomenal worlds, but I’ll set that aside for now.)
We should recognize up front that this skeptical outlook is exactly what Berkeley predicted the Lockean distinction of primary from secondary qualities would yield; if we know only how a thing appears to us, and not what it actually is, then of course we can’t infer its true powers from its appearances!
But is this right? In a brilliant recent appropriation of Aristotle, James Ross argued (in Thought and World) that our knowledge of the world depends on our ability to abstract the intelligible structures that constitute the world’s items as members in one (or many) natural kinds. My ability reliably to distinguish water from vinegar (by virtue of its smell, or perhaps its interactions with baking soda) depends on my intuitive knowledge of real attributes of these two distinct kinds, real potencies to provoke distinct responses from my nose, or (failing that) my digestive tract, or (analogously) from a beaker containing baking soda. Our perceptions aren’t just incidentally conjoined to things in themselves; they put us in touch with real properties of those things, which is why those perceptions can be intelligibly related to an increasingly complete mathematical description of the intelligible structures that render this liquid water, that one baking soda, and so on.
If, for Aristotelians, things in the world are matter variously informed by intelligible structures, then we might construe Berkeley’s innovation on Aristotle as eliminating the unnecessary hypothesis of matter — things, for Berkeley, simply are intelligible structures.