(CG 2/49). That Subsistent Intelligence is not Corporeal (Book 2: God The Origin of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

THOUGH subsistent intelligences are not corporeal, nor compounded of matter and form, nor existent as material forms in matter, still it must not be thought that they come up to the simplicity of the being of God: for there is found in them a certain composition, inasmuch as existence (esse) and essence (quod est) is not in them the same.

4. Whatsoever reality subsists of and by itself, nothing attaches to that reality except what is proper to being as being. For what is said of any reality not as such, does not belong to that reality otherwise than accidentally by reason of the subject: hence, considered apart from the subject in a particular case, the attribute does not belong to that reality at all. Now to be 'caused by another' does not belong to being, as being: otherwise every being would be caused by another, which is impossible (B. I, Chap. XIII) Therefore that existence which is being of itself and by itself, must be uncaused. No caused being therefore is its own existence.

5. The substance of every reality is a being of itself and not through another. Hence actual illumination is not of the substance of air, because it accrues to it through another. But to every created reality existence accrues through another, otherwise it would not be a creature. Therefore of no created substance is it true to say that its existence is its substance.

Hence in Exodus iii, 14, existence is assigned as the proper name of God, He who is: because it is proper to God alone that His substance is none other than His existence.