f knowledge:
secondly, into his medium of knowledge: thirdly, into the objects
known: and fourthly, into the manner whereby he knows them.

Under the first heading there are five points of inquiry:

(1) Is the angel's understanding his substance?

(2) Is his being his understanding?

(3) Is his substance his power of intelligence?

(4) Is there in the angels an active and a passive intellect?

(5) Is there in them any other power of knowledge besides the
intellect?
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FIRST ARTICLE [I, Q. 54, Art. 1]

Whether an Angel's Act of Understanding Is His Substance?

Objection 1: It would seem that the angel's act of understanding is
his substance. For the angel is both higher and simpler than the
active intellect of a soul. But the substance of the active intellect
is its own action; as is evident from Aristotle (De Anima iii) and
from his Commentator [*Averroes, A.D. 1126-1198]. Therefore much more
is the angel's substance his action--that is, his act of
understanding.

Obj. 2: Further, the Philosopher says (Metaph. xii, text 39) that
"the action of the intellect is life." But "since in living things to
live is to be," as he says (De Anima ii, text 37), it seems that life
is essence. Therefore the action of the intellect is the essence of
an angel who understands.

Obj. 3: Further, if the extremes be one, then the middle does not
differ from them; because extreme is farther from extreme than the
middle is. But in an angel the intellect and the object understood
are the same, at least in so far as he understands his own essence.
Therefore the act of understanding, which is between the intellect
and the thing understood, is one with the substance of the angel who
understands.

_On the contrary,_ The action of anything differs more from its
substance than does its existence. But no creature's existence is its
substance, for this belongs to God only, as is evident from what was
said above (Q. 3, A. 4). Therefore neither the action of an angel,
nor of any other creature, is its substance.

_I answer that,_ It is impossible for the action of an angel, or of
any creature, to be its own substance. For an action is properly the
actuality of a power; just as existence is the actuality of a
substance or of an essence. Now it is impossible for anything which
is not a pure act, but which has some admixture of potentiality, to
be its own actuality: because actuality is opposed to potentiality.
But G