Objection 1. It would seem that the angels do not know material things. For the object understood is the perfection of him who understands it. But material things cannot be the perfections of angels, since they are beneath them. Therefore the angels do not know material things.
Objection 2. Further, intellectual vision is only of such things as exist within the soul by their essence, as is said in the gloss [On 2 Corinthians 12:2, taken from Augustine (Gen. ad lit. xii. 28)]. But the material things cannot enter by their essence into man's soul, nor into the angel's mind. Therefore they cannot be known by intellectual vision, but only by imaginary vision, whereby the images of bodies are apprehended, and by sensible vision, which regards bodies in themselves. Now there is neither imaginary nor sensible vision in the angels, but only intellectual. Therefore the angels cannot know material things.
Objection 3. Further, material things are not actually intelligible, but are knowable by apprehension of sense and of imagination, which does not exist in angels. Therefore angels do not know material things.
On the contrary, Whatever the lower power can do, the higher can do likewise. But man's intellect, which in the order of nature is inferior to the angel's, can know material things. Therefore much more can the mind of an angel.
I answer that, The established order of things is for the higher beings to be more perfect than the lower; and for whatever is contained deficiently, partially, and in manifold manner in the lower beings, to be contained in the higher eminently, and in a certain degree of fulness and simplicity. Therefore, in God, as in the highest source of things, all things pre-exist supersubstantially in respect of His simple Being itself, as Dionysius says (Div. Nom. 1). But among other creatures the angels are nearest to God, and resemble Him most; hence they share more fully and more perfectly in the Divine goodness, as Dionysius says (Coel. Hier. iv). Consequently, all material things pre-exist in the angels more simply and less materially even than in themselves, yet in a more manifold manner and less perfectly than in God.
Now whatever exists in any subject, is contained in it after the manner of such subject. But the angels are intellectual beings of their own nature. Therefore, as God knows material things by His essence, so do the angels know them, forasmuch as they are in the angels by their intelligible species.
Reply to Objection 1. The thing understood is the perfection of the one who understands, by reason of the intelligible species which he has in his intellect. And thus the intelligible species which are in the intellect of an angel are perfections and acts in regard to that intellect.
Reply to Objection 2. Sense does not apprehend the essences of things, but only their outward accidents. In like manner neither does the imagination; for it apprehends only the images of bodies. The intellect alone apprehends the essences of things. Hence it is said (De Anima iii, text. 26) that the object of the intellect is "what a thing is," regarding which it does not err; as neither does sense regarding its proper sensible object. So therefore the essences of material things are in the intellect of man and angels, as the thing understood is in him who understands, and not according to their real natures. But some things are in an intellect or in the soul according to both natures; and in either case there is intellectual vision.
Reply to Objection 3. If an angel were to draw his knowledge of material things from the material things themselves, he would require to make them actually intelligible by a process of abstraction. But he does not derive his knowledge of them from the material things themselves; he has knowledge of material things by actually intelligible species of things, which species are connatural to him; just as our intellect has, by species which it makes intelligible by abstraction.
Source: Summa Theologica (New Advent)