SIXTH ARTICLE [I, Q. 41, Art. 6]
Whether Several Persons Can Be the Term of One Notional Act?
Objection 1: It would seem that a notional act can be directed to several Persons, so that there may be several Persons begotten or spirated in God. For whoever has the power of begetting can beget. But the Son has the power of begetting. Therefore He can beget. But He cannot beget Himself: therefore He can beget another son. Therefore there can be several Sons in God.
Obj. 2: Further, Augustine says (Contra Maxim. iii, 12): "The Son did not beget a Creator: not that He could not, but that it behoved Him not."
Obj. 3: Further, God the Father has greater power to beget than has a created father. But a man can beget several sons. Therefore God can also: the more so that the power of the Father is not diminished after begetting the Son.
On the contrary, In God "that which is possible," and "that which is" do not differ. If, therefore, in God it were possible for there to be several Sons, there would be several Sons. And thus there would be more than three Persons in God; which is heretical.
I answer that, As Athanasius says, in God there is only "one Father, one Son, one Holy Ghost." For this four reasons may be given.
The first reason is in regard to the relations by which alone are the Persons distinct. For since the divine Persons are the relations themselves as subsistent, there would not be several Fathers, or several Sons in God, unless there were more than one paternity, or more than one filiation. And this, indeed, would not be possible except owing to a material distinction: since forms of one species are not multiplied except in respect of matter, which is not in God. Wherefore there can be but one subsistent filiation in God: just as there could be but one subsistent whiteness.
The second reason is taken from the manner of the processions. For God understands and wills all things by one simple act. Wherefore there can be but one person proceeding after the manner of word, which person is the Son; and but one person proceeding after the manner of love, which person is the Holy Ghost.
The third reason is taken from the manner in which the persons proceed. For the persons proceed naturally, as we have said (A. 2), and nature is determined to one.