Thomas Aquinas

“ Further, if the extremes be one, then the middle does not differ from them; because extreme is farther from extreme than the middle is. But in an angel the intellect and the object understood are the same, at least in so far as he understands his own essence. Therefore the act of understanding, which is between the intellect and the thing understood, is one with the substance of the angel who understands. ”

“ It belongs to the passive intellect to be in potentiality with regard to things which are naturally capable of being known, and sometimes to apprehend them actually. Hence for one angel to enlighten another does not belong to the notion of an active intellect: neither does it belong to the passive intellect for the angel to be enlightened with regard to supernatural mysteries, to the knowledge of which he is sometimes in potentiality. ”

“ But the act of sensation is relatively infinite, for it bears relation to all sensible things; as sight does to all things visible. Now the being of every creature is restricted to one in genus and species; God's being alone is simply infinite, comprehending all things in itself, as Dionysius says (Div. Nom. v) . Hence the Divine nature alone is its own act of understanding and its own act of will. ”

“ 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. ”

“ Besides, if an angel's act of understanding were his substance, it would be necessary for it to be subsisting. Now a subsisting act of intelligence can be but one; just as an abstract thing that subsists. Consequently an angel's substance would neither be distinguished from God's substance, which is His very act of understanding subsisting in itself, nor from the substance of another angel. ”

“ Life is sometimes taken for the existence of the living subject: sometimes also for a vital operation, that is, for one whereby something is shown to be living. In this way the Philosopher says that to understand is, in a sense, to live: for there he distinguishes the various grades of living things according to the various functions of life.
Reply to Objection 2: The essence of an angel is the reason of his entire existence, but not the reason of his whole act of understanding, since he cannot understand everything by his essence. ”

“ 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 (Q [44] , A [1] ) . ”

“ 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." ”

“ The action which is transient, passing to some extrinsic object, is really a medium between the agent and the subject receiving the action. The action which remains within the agent, is not really a medium between the agent and the object, but only according to the manner of expression ”

“ Because, since sense has a sure apprehension of its proper sensible object, it is a common usage of speech, when he understands something for certain, to say that we "sense it." And hence it is that we use the word "sentence." Experience can be attributed to the angels according to the likeness of the things known, although not by likeness of the faculty knowing them. ”