s. 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 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 Obj. 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."