Reply Obj. 1: One, as it is a transcendental, is wider and more general than substance and relation. And so likewise is multitude; hence in God it may mean both substance and relation, according to the context. Still, the very signification of such names adds a negation of division, beyond substance and relation; as was explained above.
Reply Obj. 2: Multitude, which denotes something real in creatures, is a species of quantity, and cannot be used when speaking of God: unlike transcendental multitude, which adds only indivision to those of which it is predicated. Such a kind of multitude is applicable to God.
Reply Obj. 3: "One" does not exclude multitude, but division, which logically precedes one or multitude. Multitude does not remove unity, but division from each of the individuals which compose the multitude. This was explained when we treated of the divine unity (Q. 11, A. 2).
It must be observed, nevertheless, that the opposite arguments do not sufficiently prove the point advanced. Although the idea of solitude is excluded by plurality, and the plurality of gods by unity, it does not follow that these terms express this signification alone. For blackness is excluded by whiteness; nevertheless, the term whiteness does not signify the mere exclusion of blackness. _______________________
FOURTH ARTICLE [I, Q. 30, Art. 4]
Whether This Term "Person" Can Be Common to the Three Persons?
Objection 1: It would seem that this term "person" cannot be common to the three persons. For nothing is common to the three persons but the essence. But this term "person" does not signify the essence directly. Therefore it is not common to all three.
Obj. 2: Further, the common is the opposite to the incommunicable. But the very meaning of person is that it is incommunicable; as appears from the definition given by Richard of St. Victor (Q. 29, A. 3, ad 4). Therefore this term "person" is not common to all the three persons.
Obj. 3: Further, if the name "person" is common to the three, it is common either really, or logically. But it is not so really; otherwise the three persons would be one person; nor again is it so logically; otherwise person would be a universal. But in God there is neither universal nor particular; neither genus nor species, as we proved above (Q. 3, A. 5). Therefore this term 'person' is not common to the three.
On the contrary, Augustine says (De Trin. vii, 4) that when we ask, "Three what?" we say, "Three persons," because what a person is, is common to them.