Just when I think I’m getting a basic grasp on Aquinas I have to stop and rethink what I had once supposed to be true of his views. I finally picked up his Being and Essence where he comments:
Form and matter are found in composite substances, as for example soul and body in man. But it cannot be said that either one of these alone is called the essence. That the matter alone of a thing is not its essence is evident, for through its essence a thing is knowable and fixed in its species and genus. But matter is not a principle of knowledge, and a thing is not placed in a genus or species through it but through that by which a thing is actual.
And he continues:
Neither can the form alone of a composite substance be called its essence, though some want to assert this. It is evident from what has been said that the essence is what is signified through the definition of a thing. Now the definition of natural substances includes not only form but also matter…It is evident, therefore, that essence embraces both matter and form.
I found this illuminating. I had previously labored under the mistaken impression that Aquinas identifed the essence of a composite substance with its form, since the form is that by which a thing is what it is. But this, evidently, is not the case.