Summa summae S. Thomae sive compendium theologiae

Procession, as such, is nothing other than the origin of one from another; hence a divine procession is the origin of one person from another, or from two. But since every procession follows upon some action,¹ just as according to an action that tends toward external matter there is a procession ad extra, so according to an action that remains within the agent itself there is considered a certain procession ad intra. It is concerning this latter that the present question is asked: whether it exists in God.

I answer: There are processions ad intra in God.²

This is evident from all the arguments by which we previously proved the divinity of the Son and of the Holy Spirit, a divinity which they possess only insofar as it has been communicated to them by way of procession.

Moreover, it is of faith that there are in God several persons really distinct within one and the same numerical nature. Therefore there are processions in God. The consequence is proved as follows. If there were no processions in God, there would be no relative opposition of origin. With the relative opposition of origin removed, several persons could not be distinguished except by a diversity of nature. Therefore. Hence Christ, speaking of Himself, says in John vi, 42: I proceeded from God; and in the same Gospel, xv, 26, concerning the Holy Spirit: Who proceeds from the Father; and thus the Church sings in the Nicene Creed concerning the Son: Begotten of the Father, God from God; and concerning the Holy Spirit: Who proceeds from the Father and the Son.