Let’s continue to look at simple substances. In this paragraph, Thomas explains why they must be a composition between essence and existence.
Although substances of this kind are form alone and are without matter, they are nevertheless not in every way simple, and they are not pure act; rather, they have an admixture of potency, and this can be seen as follows.
Interesting. Where there is potency, there is (by definition) the possibility of change. But what kind of change?
Whatever is not in the concept of the essence or the quiddity comes from beyond the essence and makes a composition with the essence, because no essence can be understood without the things that are its parts.
If there’s something in a substance that is not in the essence, then it’s composed with the essence; if it were essential, it would be part of the essence, and if it’s not essential and not composed, it’s not part of the substance.
But every essence or quiddity can be understood without understanding anything about its existence: I can understand what a man is or what a phoenix is and nevertheless not know whether either has existence in reality.
Right. A phoenix has an essence, and thanks to Harry Potter every 10-year-old knows what it is. But there aren’t any.
Therefore, it is clear that existence is something other than the essence or quiddity, unless perhaps there is something whose quiddity is its very own existence, and this thing must be one and primary.
And that would be God. Why must this thing be one and primary?
For, there can be no plurification of something except by the addition of some difference, as the nature of a genus is multiplied in its species; or as, since the form is received in diverse matters, the nature of the species is multiplied in diverse individuals; or again as when one thing is absolute and another is received in something else, as if there were a certain separate heat that was other than unseparated heat by reason of its own separation.
OK, I got the first two cases: species differ from other species in the same genus by their specific difference; individuals within a species differ by their matter. But the emphasized phrase is not quite clear to me.
But if we posit a thing that is existence only, such that it is subsisting existence itself, this existence will not receive the addition of a difference, for, if there were added a difference, there would be not only existence but existence and also beyond this some form;
Well, OK; by definition. If it’s only existence, it’s only existence.
…much less would such a thing receive the addition of matter, for then the thing would be not subsisting existence but material existence.
This presumably ties in with the separated heat thing up above. I still don’t quite get it.
Hence, it remains that a thing that is its own existence cannot be other than one, and so in every other thing, the thing’s existence is one thing, and its essence or quiddity or nature or form is another.
If there were a thing that is only its own existence, then it is only its own existence. Thomas hasn’t really explained why there can’t be a thing that is its own existence and more.
Oh, but wait. He’s not trying to prove the existence of God, here; he’s trying to prove that angels are not utterly simple. An angelic substance exists; so it has existence. But if it’s not composite, then it has only existence; and then it must be utterly simple, and also unique, and has no form. But the intelligences have form. So their existence must be separate and composed.
In the intelligences, therefore, there is existence beyond the form, and so we say that an intelligence is form and existence.
OK, I think I’ve got that, mostly.