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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.
Obj. 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.
Obj. 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 (Q. 14, A. 2; Q. 54, A. 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