Objection 1. It would seem that the notional acts are not to be attributed to the persons. For Boethius says (De Trin.): "Whatever is predicated of God, of whatever genus it be, becomes the divine substance, except what pertains to the relation." But action is one of the ten "genera." Therefore any action attributed to God belongs to His essence, and not to a notion.
Objection 2. Further, Augustine says (De Trin. v, 4,5) that, "everything which is said of God, is said of Him as regards either His substance, or relation." But whatever belongs to the substance is signified by the essential attributes; and whatever belongs to the relations, by the names of the persons, or by the names of the properties. Therefore, in addition to these, notional acts are not to be attributed to the persons.
Objection 3. Further, the nature of action is of itself to cause passion. But we do not place passions in God. Therefore neither are notional acts to be placed in God.
On the contrary, Augustine (Fulgentius, De Fide ad Petrum ii) says: "It is a property of the Father to beget the Son." Therefore notional acts are to be placed in God.
I answer that, In the divine persons distinction is founded on origin. But origin can be properly designated only by certain acts. Wherefore, to signify the order of origin in the divine persons, we must attribute notional acts to the persons.
Reply to Objection 1. Every origin is designated by an act. In God there is a twofold order of origin: one, inasmuch as the creature proceeds from Him, and this is common to the three persons; and so those actions which are attributed to God to designate the proceeding of creatures from Him, belong to His essence. Another order of origin in God regards the procession of person from person; wherefore the acts which designate the order of this origin are called notional; because the notions of the persons are the mutual relations of the persons, as is clear from what was above explained (32, 2).
Reply to Objection 2. The notional acts differ from the relations of the persons only in their mode of signification; and in reality are altogether the same. Whence the Master says that "generation and nativity in other words are paternity and filiation" (Sent. i, D, xxvi). To see this, we must consider that the origin of one thing from another is firstly inferred from movement: for that anything be changed from its disposition by movement evidently arises from some cause. Hence action, in its primary sense, means origin of movement; for, as movement derived from another into a mobile object, is called "passion," so the origin of movement itself as beginning from another and terminating in what is moved, is called "action." Hence, if we take away movement, action implies nothing more than order of origin, in so far as action proceeds from some cause or principle to what is from that principle. Consequently, since in God no movement exists, the personal action of the one producing a person is only the habitude of the principle to the person who is from the principle; which habitudes are the relations, or the notions. Nevertheless we cannot speak of divine and intelligible things except after the manner of sensible things, whence we derive our knowledge, and wherein actions and passions, so far as these imply movement, differ from the relations which result from action and passion, and therefore it was necessary to signify the habitudes of the persons separately after the manner of act, and separately after the manner of relations. Thus it is evident that they are really the same, differing only in their mode of signification.
Reply to Objection 3. Action, so far as it means origin of movement, naturally involves passion; but action in that sense is not attributed to God. Whence, passions are attributed to Him only from a grammatical standpoint, and in accordance with our manner of speaking, as we attribute "to beget" with the Father, and to the Son "to be begotten."
Source: Summa Theologica (New Advent)