(CG 1/020). That God is Incorporeal (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

IN everything that is not its own essence, quiddity, or nature, there must be some composition. For since in everything its own essence is contained,�if in anything there were contained nothing but its essence, the whole of that thing would be its essence, and so itself would be its own essence. If then anything is not its own essence, there must be something in that thing besides its essence, and so there must be in it composition. Hence also the essence in compound things is spoken of as a part, as humanity in man. But it has been shown that in God there is no composition. God therefore is His own essence.

2. That alone is reckoned to be beyond the essence of a thing, which does not enter into its definition: for the definition declares what the thing essentially is. But the accidents of a thing are the only points about it which fall not within the definition: therefore the accidents are the only points about a thing besides its essence. But in God there are no accidents, as will be shown (Chap. XXIII): therefore there is nothing in Him besides His essence.

3. The forms that are not predicable of subsistent things, whether in the universal or in the singular, are forms that do not of themselves subsist singly, individualised in themselves. It is not said that Socrates or man or animal is whiteness; because whiteness is not anything subsisting singly in itself, but is individualised by the substance in which it exists. Also the essences or quiddities of genera or species are individualised according to the definite matter of this or that individual, although the generic or specific quiddity includes form and matter in general: hence it is not said that Socrates or man is humanity. But the Divine Essence is something existing singly by itself, and individualised in itself, as will be shown (Chap. XLII). The Divine Essence therefore is predicated of God in such a way that it can be said: 'God is His own essence.'