Objection 1. It would seem that in God there is no power in respect of the notional acts. For every kind of power is either active or passive; neither of which can be here applied, there being in God nothing which we call passive power, as above explained (25, 1); nor can active power belong to one person as regards another, since the divine persons were not made, as stated above (Article 3). Therefore in God there is no power in respect of the notional acts.
Objection 2. Further, the object of power is what is possible. But the divine persons are not regarded as possible, but necessary. Therefore, as regards the notional acts, whereby the divine persons proceed, there cannot be power in God.
Objection 3. Further, the Son proceeds as the word, which is the concept of the intellect; and the Holy Ghost proceeds as love, which belongs to the will. But in God power exists as regards effects, and not as regards intellect and will, as stated above (Question 25, Article 1). Therefore, in God power does not exist in reference to the notional acts.
On the contrary, Augustine says (Contra Maxim. iii, 1): "If God the Father could not beget a co-equal Son, where is the omnipotence of God the Father?" Power therefore exists in God regarding the notional acts.
I answer that, As the notional acts exist in God, so must there be also a power in God regarding these acts; since power only means the principle of act. So, as we understand the Father to be principle of generation; and the Father and the Son to be the principle of spiration, we must attribute the power of generating to the Father, and the power of spiration to the Father and the Son; for the power of generation means that whereby the generator generates. Now every generator generates by something. Therefore in every generator we must suppose the power of generating, and in the spirator the power of spirating.
Reply to Objection 1. As a person, according to notional acts, does not proceed as if made; so the power in God as regards the notional acts has no reference to a person as if made, but only as regards the person as proceeding.
Reply to Objection 2. Possible, as opposed to what is necessary, is a consequence of a passive power, which does not exist in God. Hence, in God there is no such thing as possibility in this sense, but only in the sense of possible as contained in what is necessary; and in this latter sense it can be said that as it is possible for God to be, so also is it possible that the Son should be generated.
Reply to Objection 3. Power signifies a principle: and a principle implies distinction from that of which it is the principle. Now we must observe a double distinction in things said of God: one is a real distinction, the other is a distinction of reason only. By a real distinction, God by His essence is distinct from those things of which He is the principle by creation: just as one person is distinct from the other of which He is principle by a notional act. But in God the distinction of action and agent is one of reason only, otherwise action would be an accident in God. And therefore with regard to those actions in respect of which certain things proceed which are distinct from God, either personally or essentially, we may ascribe power to God in its proper sense of principle. And as we ascribe to God the power of creating, so we may ascribe the power of begetting and of spirating. But "to understand" and "to will" are not such actions as to designate the procession of something distinct from God, either essentially or personally. Wherefore, with regard to these actions we cannot ascribe power to God in its proper sense, but only after our way of understanding and speaking: inasmuch as we designate by different terms the intellect and the act of understanding in God, whereas in God the act of understanding is His very essence which has no principle.
Source: Summa Theologica (New Advent)