at
which it represents cannot be said to be equal to the image." But the
Son is the image of the Father; and so the Father is not equal to the
Son. Therefore equality is not to be found among the divine persons.

Obj. 4: Further, equality is a relation. But no relation is common to
the three persons; for the persons are distinct by reason of the
relations. Therefore equality is not becoming to the divine persons.

_On the contrary,_ Athanasius says that "the three persons are
co-eternal and co-equal to one another."

_I answer that,_ We must needs admit equality among the divine persons.
For, according to the Philosopher (Metaph. x, text 15, 16, 17),
equality signifies the negation of greater or less. Now we cannot
admit anything greater or less in the divine persons; for as Boethius
says (De Trin. i): "They must needs admit a difference [namely, of
Godhead] who speak of either increase or decrease, as the Arians do,
who sunder the Trinity by distinguishing degrees as of numbers, thus
involving a plurality." Now the reason of this is that unequal things
cannot have the same quantity. But quantity, in God, is nothing else
than His essence. Wherefore it follows, that if there were any
inequality in the divine persons, they would not have the same
essence; and thus the three persons would not be one God; which is
impossible. We must therefore admit equality among the divine persons.

Reply Obj. 1: Quantity is twofold. There is quantity of "bulk" or
dimensive quantity, which is to be found only in corporeal things,
and has, therefore, no place in God. There is also quantity of
"virtue," which is measured according to the perfection of some
nature or form: to this sort of quantity we allude when we speak of
something as being more, or less, hot; forasmuch as it is more or
less, perfect in heat. Now this virtual quantity is measured firstly
by its source--that is, by the perfection of that form or nature:
such is the greatness of spiritual things, just as we speak of great
heat on account of its intensity and perfection. And so Augustine
says (De Trin. vi, 18) that "in things which are great, but not in
bulk, to be greater is to be better," for the more perfect a thing is
the better it is. Secondly, virtual quantity is measured by the
effects of the form. Now the first effect of form is being, for
everything has being by reason of its form. The second effect is
operation, for every agent acts through its form. Consequentl