In Chapter 10 we get technical; but Thomas also defines his terms better than he sometimes does. (If I ever write a book of theology, I think I’ll call it Things St. Thomas Aquinas Took For Granted.) Here he talks about the meaning of the word essence:
The further conclusion follows that God is His own essence. The essence of anything is that which its definition signifies. This is identical with the thing of which it is the definition, unless per accidens something is added to the thing defined over and above its definition. Thus whiteness is added to man, over and above the fact that he is a rational and mortal animal. Hence rational and mortal animal is the same as man; but whiteness, so far as it is white, is not the same as man.
A thing’s essence, then, is precisely that about it which is not per accidens, accidental. This is consistent with my definition of these terms in previous posts (I was reading ahead), though more clearly stated. (Go figure.)
In any being, therefore, in which there are not found two factors whereof one is per se and the other per accidens, its essence must be altogether identical with it.
This follows quite clearly. Every thing must have at least one factor which is per se, that is, essential. A circle in the plane, for example, is the set of points equidistant from some other point. That’s the essence of a circle. Take that away, and you’ve got no kind of thing at all. And if the thing has no accidents, as in the case of the geometric definition of a circle, then the definition is the thing itself.
In God, however, since He is simple, as has been shown, there are not found two factors whereof one is per se and the other per accidens. Therefore His essence must be absolutely the same as He Himself.
Aha! I think this is the point I was struggling with in earlier posts. You’ve got to have essence; you don’t have to have accidents. So if there’s only one thing, and that thing is simple, without part, then all you can have is essence. God, as Thomas has shown Him to be, is simple. So God is His own essence, His own definition. He Is.
Moreover, whenever an essence is not absolutely identical with the thing of which it is the essence, something is discerned in that thing that has the function of potency, and something else that has the function of act.
This is to say, when a thing has accidents, it cannot be pure act: it must have the potentiality to change. Why? Because the accidents are not essential, and consequently can change without changing the essence of the thing. Dye the dog blue, and he’s still a dog. The dog’s color is per accidens.
For an essence is formally related to the thing of which it is the essence as humanity is related to man. In God. however, no potency and act can be discerned: He is pure act. Accordingly He is His essence.
And God has no potential for change, being immutable; by definition, then He is pure act, and can have no accidents. Essence is all that’s left, so God is His own essence.
I like this chapter a whole lot; it shines a spotlight on a number of things that have been making less than perfect sense. In the words of a card I was given when I graduated from high school, I feel like I am now confused on a higher level, about more important things.
That said, it’s not clear why this point matters. But doubtless this will be explained in due course.