So, names
which signify the divine essence in a substantive manner are
predicated of the three persons in the singular, and not in the
plural. This, then, is the reason why we say that Socrates, Plato and
Cicero are "three men"; whereas we do not say the Father, Son and
Holy Ghost are "three Gods," but "one God"; forasmuch as in the three
_supposita_ of human nature there are three humanities, whereas in
the three divine Persons there is but one divine essence. On the
other hand, the names which signify essence in an adjectival manner
are predicated of the three persons plurally, by reason of the
plurality of _supposita._ For we say there are three "existent" or
three "wise" beings, or three "eternal," "uncreated," and "immense"
beings, if these terms are understood in an adjectival sense. But if
taken in a substantive sense, we say "one uncreated, immense, eternal
being," as Athanasius declares.

Reply Obj. 1: Though the name "God" signifies a being having Godhead,
nevertheless the mode of signification is different. For the name
"God" is used substantively; whereas "having Godhead" is used
adjectively. Consequently, although there are "three having Godhead,"
it does not follow that there are three Gods.

Reply Obj. 2: Various languages have diverse modes of expression. So
as by reason of the plurality of _supposita_ the Greeks said "three
hypostases," so also in Hebrew "Elohim" is in the plural. We,
however, do not apply the plural either to "God" or to "substance,"
lest plurality be referred to the substance.

Reply Obj. 3: This word "thing" is one of the transcendentals.
Whence, so far as it is referred to relation, it is predicated of God
in the plural; whereas, so far as it is referred to the substance, it
is predicated in the singular. So Augustine says, in the passage
quoted, that "the same Trinity is a thing supreme."

Reply Obj. 4: The form signified by the word "person" is not essence
or nature, but personality. So, as there are three
personalities--that is, three personal properties in the Father, Son
and Holy Ghost--it is predicated of the three, not in the singular,
but in the plural.
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FOURTH ARTICLE [I, Q. 39, Art. 4]

Whether the Concrete Essential Names Can Stand for the Person?

Objection 1: It would seem that the concrete, essential names cannot
stand for the person, so that we can truly say "God begot God." For,
as the logicians say, "a singular term signifies what i