Objection 1. It would seem that in an angel the power or faculty of understanding is not different from his essence. For, "mind" and "intellect" express the power of understanding. But in many passages of his writings, Dionysius styles angels "intellects" and "minds." Therefore the angel is his own power of intelligence.
Objection 2. Further, if the angel's power of intelligence be anything besides his essence, then it must needs be an accident; for that which is besides the essence of anything, we call it accident. But "a simple form cannot be a subject," as Boethius states (De Trin. 1). Thus an angel would not be a simple form, which is contrary to what has been previously said (50, 2).
Objection 3. Further, Augustine (Confess. xii) says, that God made the angelic nature "nigh unto Himself," while He made primary matter "nigh unto nothing"; from this it would seem that the angel is of a simpler nature than primary matter, as being closer to God. But primary matter is its own power. Therefore much more is an angel his own power of intelligence.
On the contrary, Dionysius says (Coel. Hier. xi) that "the angels are divided into substance, power, and operation." Therefore substance, power, and operation, are all distinct in them.
I answer that, Neither in an angel nor in any creature, is the power or operative faculty the same as its essence: which is made evident thus. Since every power is ordained to an act, then according to the diversity of acts must be the diversity of powers; and on this account it is said that each proper act responds to its proper power. But in every creature the essence differs from the existence, and is compared to it as potentiality is to act, as is evident from what has been already said (44, 1). Now the act to which the operative power is compared is operation. But in the angel to understand is not the same as to exist, nor is any operation in him, nor in any other created thing, the same as his existence. Hence the angel's essence is not his power of intelligence: nor is the essence of any creature its power of operation.
Reply to Objection 1. An angel is called "intellect" and "mind," because all his knowledge is intellectual: whereas the knowledge of a soul is partly intellectual and partly sensitive.
Reply to Objection 2. A simple form which is pure act cannot be the subject of accident, because subject is compared to accident as potentiality is to act. God alone is such a form: and of such is Boethius speaking there. But a simple form which is not its own existence, but is compared to it as potentiality is to act, can be the subject of accident; and especially of such accident as follows the species: for such accident belongs to the form--whereas an accident which belongs to the individual, and which does not belong to the whole species, results from the matter, which is the principle of individuation. And such a simple form is an angel.
Reply to Objection 3. The power of matter is a potentiality in regard to substantial being itself, whereas the power of operation regards accidental being. Hence there is no comparison.
Source: Summa Theologica (New Advent)