Objection 1. It would seem that the notional acts are understood before the properties. For the Master of the Sentences says (Sent. i, D, xxvii) that "the Father always is, because He is ever begetting the Son." So it seems that generation precedes paternity in the order of intelligence.
Objection 2. Further, in the order of intelligence every relation presupposes that on which it is founded; as equality presupposes quantity. But paternity is a relation founded on the action of generation. Therefore paternity presupposes generation.
Objection 3. Further, active generation is to paternity as nativity is to filiation. But filiation presupposes nativity; for the Son is so called because He is born. Therefore paternity also presupposes generation.
On the contrary, Generation is the operation of the person of the Father. But paternity constitutes the person of the Father. Therefore in the order of intelligence, paternity is prior to generation.
I answer that, According to the opinion that the properties do not distinguish and constitute the hypostases in God, but only manifest them as already distinct and constituted, we must absolutely say that the relations in our mode of understanding follow upon the notional acts, so that we can say, without qualifying the phrase, that "because He begets, He is the Father." A distinction, however, is needed if we suppose that the relations distinguish and constitute the divine hypostases. For origin has in God an active and passive signification--active, as generation is attributed to the Father, and spiration, taken for the notional act, is attributed to the Father and the Son; passive, as nativity is attributed to the Son, and procession to the Holy Ghost. For, in the order of intelligence, origin, in the passive sense, simply precedes the personal properties of the person proceeding; because origin, as passively understood, signifies the way to a person constituted by the property. Likewise, origin signified actively is prior in the order of intelligence to the non-personal relation of the person originating; as the notional act of spiration precedes, in the order of intelligence, the unnamed relative property common to the Father and the Son. The personal property of the Father can be considered in a twofold sense: firstly, as a relation; and thus again in the order of intelligence it presupposes the notional act, for relation, as such, is founded upon an act: secondly, according as it constitutes the person; and thus the notional act presupposes the relation, as an action presupposes a person acting.
Reply to Objection 1. When the Master says that "because He begets, He is Father," the term "Father" is taken as meaning relation only, but not as signifying the subsisting person; for then it would be necessary to say conversely that because He is Father He begets.
Reply to Objection 2. This objection avails of paternity as a relation, but not as constituting a person.
Reply to Objection 3. Nativity is the way to the person of the Son; and so, in the order of intelligence, it precedes filiation, even as constituting the person of the Son. But active generation signifies a proceeding from the person of the Father; wherefore it presupposes the personal property of the Father.
Source: Summa Theologica (New Advent)