either one subsistent, with two denominations; or one
substance divided into two imperfect substances; or a third prior
substance taken and assumed by the other two." Therefore it must not
be said that the three persons are of one substance.

_On the contrary,_ Augustine says (Contra Maxim. iii) that the word
_homoousion,_ which the Council of Nicaea adopted against the Arians,
means that the three persons are of one essence.

_I answer that,_ As above explained (Q. 13, AA. 1, 2), divine things
are named by our intellect, not as they really are in themselves, for
in that way it knows them not; but in a way that belongs to things
created. And as in the objects of the senses, whence the intellect
derives its knowledge, the nature of the species is made individual by
the matter, and thus the nature is as the form, and the individual is
the _suppositum_ of the form; so also in God the essence is taken as
the form of the three persons, according to our mode of signification.
Now in creatures we say that every form belongs to that whereof it is
the form; as the health and beauty of a man belongs to the man. But we
do not say of that which has a form, that it belongs to the form,
unless some adjective qualifies the form; as when we say: "That woman
is of a handsome figure," or: "This man is of perfect virtue." In like
manner, as in God the persons are multiplied, and the essence is not
multiplied, we speak of one essence of the three persons, and three
persons of the one essence, provided that these genitives be
understood as designating the form.

Reply Obj. 1: Substance is here taken for the "hypostasis," and not
for the essence.

Reply Obj. 2: Although we may not find it declared in Holy Writ in so
many words that the three persons are of one essence, nevertheless we
find it so stated as regards the meaning; for instance, "I and the
Father are one (John 10:30)," and "I am in the Father, and the Father
in Me (John 10:38)"; and there are many other texts of the same
import.

Reply Obj. 3: Because "nature" designates the principle of action
while "essence" comes from being [essendo], things may be said to be
of one nature which agree in some action, as all things which give
heat; but only those things can be said to be of "one essence" which
have one being. So the divine unity is better described by saying
that the three persons are "of one essence," than by saying they are
"of one nature."

Reply Obj. 4: Form, in the