Objection 1. It would seem that angels do not know singulars. For the Philosopher says (Poster. i, text. 22): "The sense has for its object singulars, but the intellect, universals." Now, in the angels there is no power of understanding save the intellectual power, as is evident from what was said above (Question 54, Article 5). Consequently they do not know singulars.
Objection 2. Further, all knowledge comes about by some assimilation of the knower to the object known. But it is not possible for any assimilation to exist between an angel and a singular object, in so far as it is singular; because, as was observed above (Question 50, Article 2), an angel is immaterial, while matter is the principle of singularity. Therefore the angel cannot know singulars.
Objection 3. Further, if an angel does know singulars, it is either by singular or by universal species. It is not by singular species; because in this way he would require to have an infinite number of species. Nor is it by universal species; since the universal is not the sufficient principle for knowing the singular as such, because singular things are not known in the universal except potentially. Therefore the angel does not know singulars.
On the contrary, No one can guard what he does not know. But angels guard individual men, according to Psalm 90:11: "He hath given His angels charge over Thee." Consequently the angels know singulars.
I answer that, Some have denied to the angels all knowledge of singulars. In the first place this derogates from the Catholic faith, which asserts that these lower things are administered by angels, according to Hebrews 1:14: "They are all ministering spirits." Now, if they had no knowledge of singulars, they could exercise no provision over what is going on in this world; since acts belong to individuals: and this is against the text of Ecclesiastes 5:5: "Say not before the angel: There is no providence."
Secondly, it is also contrary to the teachings of philosophy, according to which the angels are stated to be the movers of the heavenly spheres, and to move them according to their knowledge and will.
Consequently others have said that the angel possesses knowledge of singulars, but in their universal causes, to which all particular effects are reduced; as if the astronomer were to foretell a coming eclipse from the dispositions of the movements of the heavens. This opinion does not escape the aforesaid implications; because, to know a singular, merely in its universal causes, is not to know it as singular, that is, as it exists here and now. The astronomer, knowing from computation of the heavenly movements that an eclipse is about to happen, knows it in the universal; yet he does not know it as taking place now, except by the senses. But administration, providence and movement are of singulars, as they are here and now existing.
Therefore, it must be said differently, that, as man by his various powers of knowledge knows all classes of things, apprehending universals and immaterial things by his intellect, and things singular and corporeal by the senses, so an angel knows both by his one mental power. For the order of things runs in this way, that the higher a thing is, so much the more is its power united and far-reaching: thus in man himself it is manifest that the common sense which is higher than the proper sense, although it is but one faculty, knows everything apprehended by the five outward senses, and some other things which no outer sense knows; for example, the difference between white and sweet. The same is to be observed in other cases. Accordingly, since an angel is above man in the order of nature, it is unreasonable to say that a man knows by any one of his powers something which an angel by his one faculty of knowledge, namely, the intellect, does not know. Hence Aristotle pronounces it ridiculous to say that a discord, which is known to us, should be unknown to God (De Anima i, text. 80; Metaph. text. 15).
The manner in which an angel knows singular things can be considered from this, that, as things proceed from God in order that they may subsist in their own natures, so likewise they proceed in order that they may exist in the angelic mind. Now it is clear that there comes forth from God not only whatever belongs to their universal nature, but likewise all that goes to make up their principles of individuation; since He is the cause of the entire substance of the thing, as to both its matter and its form. And for as much as He causes, does He know; for His knowledge is the cause of a thing, as was shown above (Question 14, Article 8). Therefore as by His essence, by which He causes all things, God is the likeness of all things, and knows all things, not only as to their universal natures, but also as to their singularity; so through the species imparted to them do the angels know things, not only as to their universal nature, but likewise in their individual conditions, in so far as they are the manifold representations of that one simple essence.
Reply to Objection 1. The Philosopher is speaking of our intellect, which apprehends only by a process of abstraction; and by such abstraction from material conditions the thing abstracted becomes a universal. Such a manner of understanding is not in keeping with the nature of the angels, as was said above (55, 2, 3 ad 1), and consequently there is no comparison.
Reply to Objection 2. It is not according to their nature that the angels are likened to material things, as one thing resembles another by agreement in genus, species, or accident; but as the higher bears resemblance to the lower, as the sun does to fire. Even in this way there is in God a resemblance of all things, as to both matter and form, in so far as there pre-exists in Him as in its cause whatever is to be found in things. For the same reason, the species in the angel's intellect, which are images drawn from the Divine essence, are the images of things not only as to their form, but also as to their matter.
Reply to Objection 3. Angels know singulars by universal forms, which nevertheless are the images of things both as to their universal, and as to their individuating principles. How many things can be known by the same species, has been already stated above (55, 3, ad 3).
Source: Summa Theologica (New Advent)