I answer that, The medium through which the intellect understands, is compared to the intellect understanding it as its form, because it is by the form that the agent acts. Now in order that the faculty may be 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 it is immaterial.
Reply Obj. 3: The things which are beneath the angel, and those which are above him, are in a measure in his substance, not indeed perfectly, nor according to their own proper formality—because the angel's essence, as being finite, is distinguished by its own formality from other things—but according to some common formality. Yet all things are perfectly and according to their own formality in God's essence, as in the first and universal operative power, from which proceeds whatever is proper or common to anything. Therefore God has a proper knowledge of all things by His own essence: and this the angel has not, but only a common knowledge. _______________________
SECOND ARTICLE [I, Q. 55, Art. 2]
Whether the Angels Understand by Species Drawn from Things?
Objection 1: It would seem that the angels understand by species drawn from things. For everything understood is apprehended by some likeness within him who understands it. But the likeness of the thing existing in another is there either by way of an exemplar, so that the likeness is the cause of the thing; or else by way of an image, so that it is caused by such thing. All knowledge, then, of the person understanding must either be the cause of the object understood, or else caused by it. Now the angel's knowledge is not the cause of existing things; that belongs to the Divine knowledge alone. Therefore it is necessary for the species, by which the angelic mind understands, to be derived from things.
Obj. 2: Further, the angelic light is stronger than the light of the active intellect of the soul. But the light of the active intellect abstracts intelligible species from phantasms. Therefore the light of the angelic mind can also abstract species from sensible things. So there is nothing to hinder us from saying that the angel understands through species drawn from things.
Obj. 3: Further, the species in the intellect are indifferent to what is present or distant, except in so far as they are taken from sensible objects. Therefore, if the angel does not understand by species drawn from things, his knowledge would be indifferent as to things present and distant; and so he would be moved locally to no purpose.
On the contrary, Dionysius says (Div. Nom. vii) that the "angels do not gather their Divine knowledge from things divisible or sensible."