We’re blogging through St. Thomas Aquinas’ Compendium Theologiae, sometimes called his Shorter Summa. Find the previous posts here.
(In the previous two posts we looked at what it means for a thing to have an essence (what it is) as distinct from its accidents (the things by which it differs from other members of its species, starting with location); and also at what Thomas means by genus and species. You might want to go back and refresh your memory.)
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.
When you take away the accidents, the essence is all that’s left; and if the thing is such that it cannot have any accidents, it must be identical to its essence. It must simply be what it essentially is. I am essentially human, but I am not simply human; I am this human, sitting in this room, typing at this computer. The things by which I differ from other humans (and especially my location) are my accidents, not my essence. But a thing with no accidents has no way to be different than its essence, and so simply is its essence.
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.
But God is simple; there is no composition of two parts in Him, no room for accidents as distinct from essence. And so He must be His essence.
Thomas then looks at it another way, as usual.
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. 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.
When we do find a thing that’s a composite, we usually (always?) find a combination of actuality and potentiality. An apple is actually an apple (its essence) and thus has the potential of ripening, of rotting, of being eaten. But God, Thomas has already shown, is unchanging. There is, in Him, no potency for change, and thus he cannot be composite in this way.
Anything you see around you, wherever you’re sitting while reading this, is a composite of essence and accidents, and is therefore not identical with its essence. Surprisingly, there are beings other than God that are identical with their essence; but that comes much later in the book.