St. Thomas Aquinas, Summa Contra Gentiles 2, chaps. 52-53
 

SCG II, Chapter 52:  That in Created Subsistent Intelligences there is a Difference between Existence
and Essence

1. 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.*

2. 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.

3. 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.


SCG II, Chapter 53:  That in Created Subsistent Intelligences there is Actuality and Potentiality

1. IN whatever being there are found two elements, the one complementary to the other, the proportion of the one element to the other is as the proportion of potential to actual: for nothing is completed except by its own actuality. But in a created intelligent subsistent being there are two elements, the substance itself and the existence thereof which is not the same thing as the substance. Now that existence is the complement of the existing substance: for everything actually exists by having existence. It follows that in every one of the aforesaid substances there is a composition of actuality and potentiality.

2. What is in any being, and comes of the agent that produced it, must be the actuality of that being: for it is an agent's function to make a thing be in actuality. But, as shown above (Chap. XV), all other substances have their existence of the prime agent: indeed their being created substances consists precisely in this, that they have their existence of another. Existence itself therefore is in these created substances as a sort of actualisation of the same. But that in which actuality is received is potentiality: for actuality is such in relation to potentiality. In every created subsistent being therefore there is potentiality and actuality.