od alone is pure act. Hence only in God is His substance the
same as His existence and His action.

Besides, if an angel's act of understanding were his substance, it
would be necessary for it to be subsisting. Now a subsisting act of
intelligence can be but one; just as an abstract thing that subsists.
Consequently an angel's substance would neither be distinguished from
God's substance, which is His very act of understanding subsisting in
itself, nor from the substance of another angel.

Also, if the angel were his own act of understanding, there could then
be no degrees of understanding more or less perfectly; for this comes
about through the diverse participation of the act of understanding.

Reply Obj. 1: When the active intellect is said to be its own action,
such predication is not essential, but concomitant, because, since
its very nature consists in act, instantly, so far as lies in itself,
action accompanies it: which cannot be said of the passive intellect,
for this has no actions until after it has been reduced to act.

Reply Obj. 2: The relation between "life" and "to live" is not the
same as that between "essence" and "to be"; but rather as that
between "a race" and "to run," one of which signifies the act in the
abstract, and the other in the concrete. Hence it does not follow,
if "to live" is "to be," that "life" is "essence." Although life is
sometimes put for the essence, as Augustine says (De Trin. x),
"Memory and understanding and will are one essence, one life": yet it
is not taken in this sense by the Philosopher, when he says that "the
act of the intellect is life."

Reply Obj. 3: The action which is transient, passing to some
extrinsic object, is really a medium between the agent and the
subject receiving the action. The action which remains within the
agent, is not really a medium between the agent and the object, but
only according to the manner of expression; for it really follows the
union of the object with the agent. For the act of understanding is
brought about by the union of the object understood with the one who
understands it, as an effect which differs from both.
_______________________

SECOND ARTICLE [I, Q. 54, Art. 2]

Whether in the Angel to Understand Is to Exist?

Objection 1: It would seem that in the angel to understand is to
exist. For in living things to live is to be, as the Philosopher says
(De Anima ii, text. 37). But to "understand is in a sense to live"
(De An