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