The Angelic realm is dauntingly beyond our power to comprehend. We cannot dare to plunge, or should I say, soar into the depths of the spiritual realm without sure guidance. Fortunately we have grace, and St. Thomas Aquinas. St. Thomas, as in everything he does, treats of angels and angelic knowledge thoroughly and succinctly. However, since most of us lack the subtlety of St. Thomas, we tend to pass over some important truths that are perhaps not contained explicitly in the article. It is only with difficulty and lengthy study that we find all that this great teacher was intending when he wrote. Such is the case with the current topic. I am saying nothing that new, and by no means am I furthering the thought of St. Thomas' treatment of angels. I am merely emphasizing a truth that was already present, but that we may miss the first time reading it.
Angelic knowledge is possibly the most difficult and most essential part of the study of the angles. If one does not discuss the mode of knowing for subsisting intellects, there is not much left to discuss. However, the difficulty is trying to understand beings that in a way are like us, and in another way are completely beyond us. For instance, the topic of this article, how is it that these glorious beings consider the actions of men? Are they constantly present among us, watching? Or perhaps this is superstitious medieval fancy, and they know us only because they reason to our existence from their own. Both of these accounts are lacking, but hold some element of the truth. As will be shown, angels, know individual human acts through intelligible species handed down to them by God.
We shall proceed by following, for the most part, St. Thomas' own order. As my topic is far more narrow I will of course skip those articles that are not to my purpose, and go back if there is some item that we might have left behind in our haste. First we will consider the medium of angelic knowledge. Then, skipping angelic knowledge of immaterial things, we will proceed directly to the knowledge of material things. There will then be a brief discussion of human actions in general and then the argument for the thesis. Lest we fall prey to the very thing this article is designed to remedy, we will then take a brief moment to dwell on the consequences of the preceding arguments.
A medium of knowledge is the species through which an intellect knows some thing. Now in man, as is seen in Book III of Aristotle's De Anima, knowledge is had through a universal species that is abstracted from a phantasm in the imagination, present there from an object sensed. This cannot be so in angels. Angels are held to be pure intellects, i.e. separated forms that are their own natures (Summa Theologiae, pp. Q. 50 a1,2,4). Consequently, the medium for angelic knowledge must be more carefully examined.
That the angel might be distinguished clearly from God, it should be made clear that he does not know all things through his own substance. For the angel possesses a particular substance, and this substance does not contain the perfection of being in itself. As the Angelic Doctor says himself,

However, the essence of the angel does not comprehend in itself all things, since the existence of a essence is determined to some genus and species. But this is proper to the divine essence, that is infinite in itself simply and comprehends the perfections of all things. Therefore, only God knows all things through his essence.1

Therefore, it is clear that the angels must know by some other medium, and that their intellects must be perfected by some species in order to know. (Summa Theologiae, pp. Q.55, a1)
Already, from what has been said, we can see that it is not through some abstracted species that an angel knows. As St. Thomas says, “The species through which an angel understands is not taken from things but is connatural to them.”(ST, pp Q55, a2) St. Thomas argues that human souls, since they are united to matter, are perfected through matter, otherwise their union with matter would be in vain. However, angels are not in matter in any way. Therefore, in order for angels to be perfected by some intelligible species, it must come from an intelligible “efflux,” that comes from God directly (ST, pp Q55, a2). This is not surprising insofar as God is the cause of being and perfection of all things (ST, pp Q3, 7). Therefore, God as he is causing things in existence, is at the same time informing the angels with the intelligible species of the existing things.
Now, St. Thomas says that the species of things are connatural to the angels. It is not that the species come from the very nature of the angel, it is that the nature of the angel requires species to obtain its perfection. Note that St. Thomas makes his argument through perfection, but he is not speaking of the supernatural perfection of eternal beatitude, but simply the perfections which belong to a thing. For example, if God were to create a lion and put no other animals on earth, this would be highly unfitting. To eat meat belongs to the nature of the lion. Therefore, it is connatural to the lion that there be some animal to eat, otherwise the nature is in vain and cannot obtain its natural perfection. So also, the angel must have some intelligible species granted to them by God directly, otherwise they can in no way obtain their natural perfection and there nature is in vain.
It remains then to see how angels are able to know singulars. Necessarily then, we must take under consideration things in matter. St. Thomas argues from the order of things in nature. The superior beings are more perfect than the inferior and contain the perfections of the inferior eminently, wholly and simply. This is as much to say, anything a man can do, an angel can do better. There is a gradation in the order of things. God is the source of all things and all perfections. The angels are nearest to God in the order of being. St. Thomas then says,

Thus, therefore, all material things are in angels, preexisting simply. Indeed and more immaterially than in things themselves; but more multiplied and more imperfect than they exist in God.1

St. Thomas then concludes, in the same place, that since God knows material things, then the angels know them by some intelligible species given by God.
It is important to emphasize that, while God is the cause of being of things, and thus it is clear that creatures exist in Him above all things, the angels are not a cause of our being. Their knowledge of us is given to them directly by God, not as though they are necessary agents of His causality,2 but because in order for these creatures to know it must come from God, not from creatures simply. See how clearly St. Thomas puts it in his reply to the first back in Question 55, article 2,

In the mind of an angel there are similitudes of creatures, not indeed, as taken from the creature, but from God, who is the cause of creatures and in which the first similitudes of things exist. Whence Augustine says in [The Literal Interpretation of Genesis] that just as the ratio by which a creature is fashioned, first is in the Word of God before it is fashioned in the creature, so the same ratio is made first in the intellectual creatures [i.e. the Angels] and then is itself fashioned in the creature.

We can now consider how angels know singulars. St. Thomas begins his discussion of this question by addressing two common errors. The first of which is the outright denial of knowledge of singulars in angels. This is against the Catholic faith, which holds that angels minster to individual humans; these we call guardian angels. Others have stated that angels know singulars through universal causes. St. Thomas shows that this does not end the difficulty, because to know through universal causes is not the same as to know the thing in the here and now. He gives the example of the astronomer knowing through causes that an eclipse is going to happen, but does not know it in its singularity unless he senses it.(Summa Theologiae, pp Q57, a 2)
St. Thomas again appeals to the superiority of angels to show that they must know singulars if we do, since there is a certain perfection in knowing singulars in the here and now. He therefore argues that God causes all things, not just universally, but in their very individuality. For he is the cause of each individual substance. Therefore, even the very individual must exist in God preeminently. Socrates, exists in God first and foremost. Thus the angels know Socrates in his very singularity through a species given by God. (Summa Theologiae, pp Q57, a 2)
Human actions are caused by God's agency through the will. Human actions begin in deliberation. This deliberation is about the means to the final end which belongs to man by nature. The deliberation necessarily results in choice.3 That choice terminates in action. It is important to realize, in order to avoid the foolish Pelagian error, that God must be the cause of any and all actions because there is in fact some being there. Moreover, that the first movements of deliberation are caused by God's direct action upon the human will There is no human action without God's causing it immediately, just as there is no being of any sort without God's causing it directly.
We are left, therefore, with the question of how angels can know these particular actions of men. Remember above what St. Thomas concludes about angelic knowledge of singulars, that it is precisely the thing its singularity, the here and now, that the angel knows. This follows for all kinds of being caused by God. As was discussed above, God is the immediate cause of being of every action. Therefore, the angel knows individual actions of men as made known to them by God himself, through some intelligible species.
There is an inclination is to say that angels must in some way make the individual actions of man present to himself in some immaterial way, even though it cannot be asserted how it is done. This view supported by considering the way demons are able to judge human thoughts by outward appearances. For as Augustine says demons “sometimes are able with the greatest faculty learn man's dispositions, not only through speech, but also as conceived in thought when the soul expresses them by certain signs in the body” (De Divinatione Daemonum). It seems therefore that the angels (and demons) are able to somehow consider man's actions directly. Moreover, to say that the knowledge is from some intelligible species of the individual it seems that one necessarily falls into some form of determinism. Angels would be able to conclude to individual actions from the intelligible forms of individuals. However, as St. Thomas argues, angels are unable to know that secret thoughts of men. For the will is moved by God alone and directly (Summa Theologiae, pp Q. 57, a 4). Therefore angels cannot perceive the principle of human action, so likewise, they cannot conclude to human actions from intelligible forms of individuals. Therefore, it seems that angels must in some way understand the individual human action from the man's action directly and since there is no form to abstract, the angel is not knowing it through abstraction, and thus this does not contradict what was said before.
The source of this difficulty seems to arise from the mode of human knowing. We know particulars directly from the particulars as sensed. This seems to us the most direct and simple way to know things. However, like most things man does, this is backwards. God knows things perfectly since He knows things in the order of being and through the cause of being, i.e. Himself. Angels, even naturally speaking are much more like to God than we are, and also know according to the mode of being. As was shown above, they cannot abstract. There is no agent or passive intellect, nor imagination in which to hold a phantasm.
The solution lies in this: God is the cause of being for every being, even accidental being. Therefore, He is even the cause of the being of actions. Therefore, as He is causing the action to be, sense there is some new ens, qualified though it may be, he reveals this to the angel through an intelligible species. Not that the angel sees this particular action as necessarily resulting from what is to be Socrates, but rather, since there is this particular man Socrates, he must necessarily act in some particular way here and now. The ens Socrates, is the occasion for the ens running, or cutting in Socrates. The angel cannot see into the deliberation of Socrates, but he is able know the action of Socrates as caused by God directly.
This is a mind blowing and staggering conclusion. We can imagine that an angel must be flooded with virtually infinite number of intelligible species as every single creature in the world, and every accidental being belonging to it (list the categories of a given moment) at every moment. However, this not simply the case. St. Thomas shows that an angel cannot understand many things at the same time. For the unity of an operation requires the unity of the object. Angels cannot know all things at once, because they are not infinite. Therefore they are only able to have one object for the operation of their vis intellectiva. To fill this out we can look at St. Thomas' consideration of whether the higher angels understand by fewer species. He argues that since God understands all things through himself in an absolute unity, inferior creatures know through many. Thus, those that are closer to Him know must by so much the fewer species understand all intelligible things. Therefore, his intelligible species are more universal (Summa Theologiae pp Q55, a3). Angels, therefore are not then attentive to all particulars accept in so far as the fall under the account of the universal species that they are considering.
Let us consider some speculative corollaries of this to help put the magnificence of this mode of knowing into perspective. Some have suggested that the highest angels are able to understand all creatures through the single species, created being. This does not seem unlikely especially considering what Dionysius says about the first hierarchy of angels in Chapter 7 of The Celestial Hierarchy.

They [Seraphim, Cherubim and Thrones] are perfect then not because of an enlightened understanding which enables them to analyze the many sacred things, but rather because of a primary and supreme deification, a transcendent and angelic understanding of God's work. They have been directed hierarchically not through other holy beings but directly from God Himself and they have achieved this thanks to the capacity which compared to others is the mark of their superior order. Hence they are found next to perfect and unfailing purity and are led, as permitted, into contemplation regarding the immaterial and intellectual splendor.

Dionysius is here speaking of the beatified angels, but what is most telling is that he attributes their proximity to God to “the capacity which compared to others is the mark of their superior order.” It is not hard to imagine, then, that the higher angels can take up the entire universe in a single glance;that they are so powerful that the universe itself is able to be a locus of their power. Through the one species, created being, they understand the natures of every created being now in existence. Thus, the spiritual realm is vastly beyond our own meagre material existence.
In light of the above it would behoove us to take a brief look at angelic intelligible species. Intelligible species is not said univocally of man and angels, but analogously. First, consider that every intelligible species we possess as with it the account of abstracted, and therefore universal. Angels labour under no such deficiency. They do not require that something be made universal in order for it to be understood. They understand it precisely in its particularity. The greatest angels are able to grasp Socrates-ness through the species created being.
We therefore see that angels are subsisting intellects. From their nature they must receive intelligible species connatural to them from God directly. These species are of all created being, even accidental being. Therefore it is through these species handed down by God that they understand human actions in the here and now.

1 Ipsa autem essentia angeli non comprehendit in se omnia, cum sit essentia determinata ad genus et ad speiciem. Hoc autem proprium est essentiae divinae, quae infinita est, ut in se simpliciter omnia comprehendat perfecte. Et ideo solus Deus cognoscit omnia per suam essentiam (Summa Theologiae pp Q. 55a 1). Though an interesting question, we will leave aside what is meant by an angel being determined to a genus and species in this article.

2 Sic igitur omnia materialia in ipsis angelis praeexistunt, simplicius quidem et immaterialius quam in ipsis rebus; multiplicius autem et imperfectus quam in deo. (Summa Theologiae, pp. Q57, a 1)

3 This is not to say that God cannot use angels in as a mediation for His direct causality, but it is not necessary. Furthermore, it seems that in certain ways that God cannot use the angels to cause at all, such as in the case of esse. We will avoid this digression.

4 Assuming that the deliberation comes to an end. One might be able to imagine a case where someone is in the grocery store indefinitely, but we try to dwell on the least part.