We’re blogging through St. Thomas Aquinas’ Compendium Theologiae, sometimes called his Shorter Summa. Find the previous posts here.
Having noted that God is His essence, Thomas continues,
God’s essence cannot be other than His existence. In any being whose essence is distinct from its existence, what it is must be distinct from that whereby it is. For in virtue of a thing’s existence we say that it is, and in virtue of its essence we say what it is. This is why a definition that signifies an essence manifests what a thing is. In God, however, there is no distinction between what He is and that whereby He is, since there is no composition in Him, as has been shown. Therefore God’s essence is nothing else than His existence.
God is infinite, which is to say unbounded; our finite human intellects cannot encompass Him. Thus, although He is simple in Himself, having no separable pieces, we have to speak of Him under multiple categories. We speak of His essence, of His existence, of His goodness, of His truth; but really, in all of this, we are speaking solely of Him in one way in which He makes Himself known to us.
Given this, it shouldn’t be surprising that God’s essence is His existence; or, to put it another way, that it is His essence to exist. After all, everything we can say about Him is really just the one thing.
But above, Thomas is making a more specific argument than that. An apple is an apple, with the essence of an apple; but this apple on the desk in front of me adds to the essence of an apple the real act of existence. This is a kind of composition, of separation into parts. And this is true for any kind of thing that can come to be. But we’ve already shown that there is no composition in God, nor does He come to be. Therefore, since He exists, and He is simple, His essence must be to exist.
Thomas puts a lot of weight on this conclusion; there’s something special about it, and what it is comes out in the next paragraph, which I’ll take a little at a time.
Likewise, we have proved that God is pure act without any admixture of potentiality.
Accordingly His essence must be the ultimate act in Him; for any act that has a bearing on the ultimate act, is in potency to that ultimate act.
What does Thomas mean by ultimate act?
In Thomas’ philosophy, as in Aristotle’s, every change has four causes: the material cause (that which changes), the formal cause (that which it changes into), the efficient cause (that which triggers the change), and the final cause, or goal.
In material things, the final cause is often rather trivial; stones have no desires. In persons, beings with intellect and will, the final cause may be rather remote from the current change: for example, I pack a lunch to take to work, so that I can eat lunch at lunch time, so that I’ll be able to think clearly during the afternoon and won’t get hungry before dinner, and ultimately, so that I don’t die of starvation.
So when God acts, what the ultimate end of that chain of final causality?
But the ultimate act is existence itself, ipsum esse. For, since all motion is an issuing forth from potency to act, the ultimate act must be that toward which all motion tends; and since natural motion tends to what is naturally desired, the ultimate act must be that which all desire. This is existence.
The ultimate act, the ultimate end, is existence: so says Thomas. And it’s hard to argue with that. Certainly, I eat, ultimately, so that I can go on living, so that I can remain in this state of existence.
Consequently the divine essence, which is pure and ultimate act, must be existence itself, ipsum esse.
And if everything that is tends ultimately to existence as its ultimate act, how much more so for God, whose perfections are stamped on everything that is?
Aristotle wrote a book called the Categories, which is all about the different ways there are to exist, to be. But the most basic thing is being itself. And that, Thomas has just shown, is God. He is ipsum esse, existence itself.