t be compared to essence, if the latter is a
distinct reality, as actuality to potentiality. Therefore, since in
God there is no potentiality, as shown above (A. 1), it follows
that in Him essence does not differ from existence. Therefore His
essence is His existence. Thirdly, because, just as that which has
fire, but is not itself fire, is on fire by participation; so that
which has existence but is not existence, is a being by participation.
But God is His own essence, as shown above (A. 3); if, therefore, He
is not His own existence He will be not essential, but participated
being. He will not therefore be the first being--which is absurd.
Therefore God is His own existence, and not merely His own essence.

Reply Obj. 1: A thing that has nothing added to it can be of
two kinds. Either its essence precludes any addition; thus, for
example, it is of the essence of an irrational animal to be without
reason. Or we may understand a thing to have nothing added to it,
inasmuch as its essence does not require that anything should be added
to it; thus the genus animal is without reason, because it is not of
the essence of animal in general to have reason; but neither is it to
lack reason. And so the divine being has nothing added to it in the
first sense; whereas universal being has nothing added to it in the
second sense.

Reply Obj. 2: "To be" can mean either of two things. It may
mean the act of essence, or it may mean the composition of a
proposition effected by the mind in joining a predicate to a subject.
Taking "to be" in the first sense, we cannot understand God's
existence nor His essence; but only in the second sense. We know that
this proposition which we form about God when we say "God is," is
true; and this we know from His effects (Q. 2, A. 2).
______________________

FIFTH ARTICLE [I, Q. 3, Art. 5]

Whether God Is Contained in a Genus?

Objection 1: It seems that God is contained in a genus. For a
substance is a being that subsists of itself. But this is especially
true of God. Therefore God is in a genus of substance.

Obj. 2: Further, nothing can be measured save by something of its
own genus; as length is measured by length and numbers by number. But
God is the measure of all substances, as the Commentator shows
(Metaph. x). Therefore God is in the genus of substance.

_On the contrary,_ In the mind, genus is prior to what it contains. But
nothing is prior to God either really or mentally. Therefore