Reply Obj. 4: It may be said that God has a rational nature, if reason be taken to mean, not discursive thought, but in a general sense, an intelligent nature. But God cannot be called an "individual" in the sense that His individuality comes from matter; but only in the sense which implies incommunicability. "Substance" can be applied to God in the sense of signifying self-subsistence. There are some, however, who say that the definition of Boethius, quoted above (A. 1), is not a definition of person in the sense we use when speaking of persons in God. Therefore Richard of St. Victor amends this definition by adding that "Person" in God is "the incommunicable existence of the divine nature." _______________________
FOURTH ARTICLE [I, Q. 29, Art. 4]
Whether This Word "Person" Signifies Relation?
Objection 1: It would seem that this word "person," as applied to God, does not signify relation, but substance. For Augustine says (De Trin. vii, 6): "When we speak of the person of the Father, we mean nothing else but the substance of the Father, for person is said in regard to Himself, and not in regard to the Son."
Obj. 2: Further, the interrogation "What?" refers to essence. But, as Augustine says: "When we say there are three who bear witness in heaven, the Father, the Word, and the Holy Ghost, and it is asked, Three what? the answer is, Three persons." Therefore person signifies essence.
Obj. 3: According to the Philosopher (Metaph. iv), the meaning of a word is its definition. But the definition of "person" is this: "The individual substance of the rational nature," as above stated. Therefore "person" signifies substance.
Obj. 4: Further, person in men and angels does not signify relation, but something absolute. Therefore, if in God it signified relation, it would bear an equivocal meaning in God, in man, and in angels.
On the contrary, Boethius says (De Trin.) that "every word that refers to the persons signifies relation." But no word belongs to person more strictly than the very word "person" itself. Therefore this word "person" signifies relation.
I answer that, A difficulty arises concerning the meaning of this word "person" in God, from the fact that it is predicated plurally of the Three in contrast to the nature of the names belonging to the essence; nor does it in itself refer to another, as do the words which express relation.
Hence some have thought that this word "person" of itself expresses absolutely the divine essence; as this name "God" and this word "Wise"; but that to meet heretical attack, it was ordained by conciliar decree that it was to be taken in a relative sense, and especially in the plural, or with the addition of a distinguishing adjective; as when we say, "Three persons," or, "one is the person of the Father, another of the Son," etc. Used, however, in the singular, it may be either absolute or relative. But this does not seem to be a satisfactory explanation; for, if this word "person," by force of its own signification, expresses the divine essence only, it follows that forasmuch as we speak of "three persons," so far from the heretics being silenced, they had still more reason to argue. Seeing this, others maintained that this word "person" in God signifies both the essence and the relation. Some of these said that it signifies directly the essence, and relation indirectly, forasmuch as "person" means as it were "by itself one" [per se una]; and unity belongs to the essence. And what is "by itself" implies relation indirectly; for the Father is understood to exist "by Himself," as relatively distinct from the Son. Others, however, said, on the contrary, that it signifies relation directly; and essence indirectly; forasmuch as in the definition of "person" the term nature is mentioned indirectly; and these come nearer to the truth.