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.
Objection 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.
Objection 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."
I answer that, The species whereby the angels understand are not drawn from things, but are connatural to them. For we must observe that there is a similarity between the distinction and order of spiritual substances and the distinction and order of corporeal substances. The highest bodies have in their nature a potentiality which is fully perfected by the form; whereas in the lower bodies the potentiality of matter is not entirely perfected by the form, but receives from some agent, now one form, now another. In like fashion also the lower intellectual substances --that is to say, human souls--have a power of understanding which is not naturally complete, but is successively completed in them by their drawing intelligible species from things. But in the higher spiritual substances--that is, the angels--the power of understanding is naturally complete by intelligible species, in so far as they have such species connatural to them, so as to understand all things which they can know naturally.
The same is evident from the manner of existence of such substances. The lower spiritual substances--that is, souls--have a nature akin to a body, in so far as they are the forms of bodies: and consequently from their very mode of existence it behooves them to seek their intelligible perfection from bodies, and through bodies; otherwise they would be united with bodies to no purpose. On the other hand, the higher substances--that is, the angels--are utterly free from bodies, and subsist immaterially and in their own intelligible nature; consequently they attain their intelligible perfection through an intelligible outpouring, whereby they received from God the species of things known, together with their intellectual nature. Hence Augustine says (Gen. ad lit. ii, 8): "The other things which are lower than the angels are so created that they first receive existence in the knowledge of the rational creature, and then in their own nature."
Reply to Objection 1. There are images of creatures in the angel's mind, not, indeed derived from creatures, but from God, Who is the cause of creatures, and in Whom the likenesses of creatures first exist. Hence Augustine says (Gen. ad lit. ii, 8) that, "As the type, according to which the creature is fashioned, is in the Word of God before the creature which is fashioned, so the knowledge of the same type exists first in the intellectual creature, and is afterwards the very fashioning of the creature."
Reply to Objection 2. To go from one extreme to the other it is necessary to pass through the middle. Now the nature of a form in the imagination, which form is without matter but not without material conditions, stands midway between the nature of a form which is in matter, and the nature of a form which is in the intellect by abstraction from matter and from material conditions. Consequently, however powerful the angelic mind might be, it could not reduce material forms to an intelligible condition, except it were first to reduce them to the nature of imagined forms; which is impossible, since the angel has no imagination, as was said above (Question 54, Article 5). Even granted that he could abstract intelligible species from material things, yet he would not do so; because he would not need them, for he has connatural intelligible species.
Reply to Objection 3. The angel's knowledge is quite indifferent as to what is near or distant. Nevertheless his local movement is not purposeless on that account: for he is not moved to a place for the purpose of acquiring knowledge, but for the purpose of operation.
Source: Summa Theologica (New Advent)