Objection 1. It would seem that an angel does not know himself. For Dionysius says that "the angels do not know their own powers" (Coel. Hier. vi). But, when the substance is known, the power is known. Therefore an angel does not know his own essence.
Objection 2. Further, an angel is a single substance, otherwise he would not act, since acts belong to single subsistences. But nothing single is intelligible. Therefore, since the angel possesses only knowledge which is intellectual, no angel can know himself.
Objection 3. Further, the intellect is moved by the intelligible object: because, as stated in De Anima iii, 4 understanding is a kind of passion. But nothing is moved by or is passive to itself; as appears in corporeal things. Therefore the angel cannot understand himself.
On the contrary, Augustine says (Gen. ad lit. ii) that "the angel knew himself when he was established, that is, enlightened by truth."
I answer that, As is evident from what has been previously said (14, 2; 54, 2), the object is on a different footing in an immanent, and in a transient, action. In a transient action the object or matter into which the action passes is something separate from the agent, as the thing heated is from what gave it heat, and the building from the builder; whereas in an immanent action, for the action to proceed, the object must be united with the agent; just as the sensible object must be in contact with sense, in order that sense may actually perceive. And the object which is united to a faculty bears the same relation to actions of this kind as does the form which is the principle of action in other agents: for, as heat is the formal principle of heating in the fire, so is the species of the thing seen the formal principle of sight to the eye.
It must, however, be borne in mind that this image of the object exists sometimes only potentially in the knowing faculty; and then there is only knowledge in potentiality; and in order that there may be actual knowledge, it is required that the faculty of knowledge be actuated by the species. But if it always actually possesses the species, it can thereby have actual knowledge without any preceding change or reception. From this it is evident that it is not of the nature of knower, as knowing, to be moved by the object, but as knowing in potentiality. Now, for the form to be the principle of the action, it makes no difference whether it be inherent in something else, or self-subsisting; because heat would give forth heat none the less if it were self-subsisting, than it does by inhering in something else. So therefore, if in the order of intelligible beings there be any subsisting intelligible form, it will understand itself. And since an angel is immaterial, he is a subsisting form; and, consequently, he is actually intelligible. Hence it follows that he understands himself by his form, which is his substance.
Reply to Objection 1. That is the text of the old translation, which is amended in the new one, and runs thus: "furthermore they," that is to say the angels, "knew their own powers": instead of which the old translation read--"and furthermore they do not know their own powers." Although even the letter of the old translation might be kept in this respect, that the angels do not know their own power perfectly; according as it proceeds from the order of the Divine Wisdom, Which to the angels is incomprehensible.
Reply to Objection 2. We have no knowledge of single corporeal things, not because of their particularity, but on account of the matter, which is their principle of individuation. Accordingly, if there be any single things subsisting without matter, as the angels are, there is nothing to prevent them from being actually intelligible.
Reply to Objection 3. It belongs to the intellect, in so far as if is in potentiality, to be moved and to be passive. Hence this does not happen in the angelic intellect, especially as regards the fact that he understands himself. Besides the action of the intellect is not of the same nature as the action found in corporeal things, which passes into some other matter.
Source: Summa Theologica (New Advent)