The following quote comes from Wolfgang Smith’s The Quantum Enigma:

As Aristotle pointed out long ago, the act of knowing consists in a certain union of the intellect with its object. But how can the intellect be joined to the external thing? Such a union, clearly, can only be conceived in terms of a third entity or common element, which object and subject can both possess, each in its own appropriate mode; and it just be this tertium quid, precisely, that renders the object knowable.

But only in part! For it is not, after all, the external object — lock, stock and barrel — that “passes into the subject”, but only what I have termed the tertium quid. This “third factor”, moreover, answers to the question “What?”: it is what we know. And yet it does not simply coincide with the object as such, for as just noted, the latter is perforce “more” than the tertium quid.

Now the tertium quid, to be sure, is none other than the Aristotelian morphe, the form or quiddity of the existing thing. But inasmuch as the thing does not coincide with its morphe, one needs to postulate a second principle — an X, if you will — that distinguishes the two, or makes up for the difference, so to speak. And this X — which is perforce unknowable and had no quiddity — is evidently tantamount to materia. One arrives this, by way of epistemological considerations of a rather simple kind, at the basic conceptions of the hylomorphic paradigm.