FIRST ARTICLE [I, Q. 54, Art. 1]
Whether an Angel's Act of Understanding Is His Substance?
Objection 1: It would seem that the angel's act of understanding is his substance. For the angel is both higher and simpler than the active intellect of a soul. But the substance of the active intellect is its own action; as is evident from Aristotle (De Anima iii) and from his Commentator [*Averroes, A.D. 1126-1198]. Therefore much more is the angel's substance his action—that is, his act of understanding.
Obj. 2: Further, the Philosopher says (Metaph. xii, text 39) that "the action of the intellect is life." But "since in living things to live is to be," as he says (De Anima ii, text 37), it seems that life is essence. Therefore the action of the intellect is the essence of an angel who understands.
Obj. 3: 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.
On the contrary, The action of anything differs more from its substance than does its existence. But no creature's existence is its substance, for this belongs to God only, as is evident from what was said above (Q. 3, A. 4). Therefore neither the action of an angel, nor of any other creature, is its substance.
I answer that, It is impossible for the action of an angel, or of any creature, to be its own substance. For an action is properly the actuality of a power; just as existence is the actuality of a substance or of an essence. Now it is impossible for anything which is not a pure act, but which has some admixture of potentiality, to be its own actuality: because actuality is opposed to potentiality. But God alone is pure act. Hence only in God is His substance the same as His existence and His action.
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.
Also, if the angel were his own act of understanding, there could then be no degrees of understanding more or less perfectly; for this comes about through the diverse participation of the act of understanding.
Reply Obj. 1: When the active intellect is said to be its own action, such predication is not essential, but concomitant, because, since its very nature consists in act, instantly, so far as lies in itself, action accompanies it: which cannot be said of the passive intellect, for this has no actions until after it has been reduced to act.