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perfectly completed by the form, it is necessary for all things to which the faculty extends to be contained under the form. Hence it is that in things which are corruptible, the form does not perfectly complete the potentiality of the matter: because the potentiality of the matter extends to more things than are contained under this or that form. But the intellective power of the angel extends to understanding all things: because the object of the intellect is universal being or universal truth. The angel's essence, however, does not comprise all things in itself, since it is an essence restricted to a genus and species. This is proper to the Divine essence, which is infinite, simply and perfectly to comprise all things in Itself. Therefore God alone knows all things by His essence. But an angel cannot know all things by his essence; and his intellect must be perfected by some species in order to know things.
Reply Obj. 1: When it is said that the angel knows things according to his own nature, the words "according to" do not determine the medium of such knowledge, since the medium is the similitude of the thing known; but they denote the knowing power, which belongs to the angel of his own nature.
Reply Obj. 2: As the sense in act is the sensible in act, as stated in _De Anima_ ii, text. 53, not so that the sensitive power is the sensible object's likeness contained in the sense, but because one thing is made from both as from act and potentiality: so likewise the intellect in act is said to be the thing understood in act, not that the substance of the intellect is itself the similitude by which it understands, but because that similitude is its form. Now, it is precisely the same thing to say "in things which are without matter, the intellect is the same thing as the object understood," as to say that "the intellect in act is the thing understood in act"; for a thing is actually understood, precisely because