"It is characteristic of a spirit, whether corporeal or incorporeal, always to proceed. Wherefore, even according to philosophers, an incorporeal spirit, when it proceeds from an intellect that acts through the will, conveys to its effects the forms of the active intelligence; for example, the spirit of an artificer which proceeds from his mind conveys, while it proceeds, the forms of art to the artificer’s hands, to his axe and hatchet, and to the stones and beams he is working with. This is the bearing of the statement in Wisdom 1:7: ‘The Spirit of the Lord hath filled the whole world, and that which containeth all things’; and in Job 26:13 we read: ‘His Spirit hath adorned the heavens’; and Psalm 32:6: ‘By the word of the Lord the heavens were established; and all the power of them by the Spirit of His mouth.’ Similarly it is characteristic of love, whether spiritual or carnal, always to proceed and flow and never to remain still. For this reason Chrysostom says that when the Holy Spirit has entered into the heart of a man, He flows more copiously than any fountain and does not stand still, but progresses. And John 7:38f.: ‘He that believeth in Me, as the Scripture saith: Out of his belly shall flow rivers of living water.’ And the Evangelist adds: ‘Now this He said of the Spirit.’
And so Dionysius says that divine love causes ecstasy, that is, transports: for it transports the lover into the beloved and does not allow him to remain in himself. Hence even grammarians say that the verb amo [I love] is a word indicating impetuous transition. Therefore, since the Holy Spirit is a Spirit and is spirated love, it is proper for Him simply to proceed; while it is not proper for one who is generated to proceed, but to exist in the nature received.
Hence the solution of the first objection is obvious: Procession signifies diffusion and, as it were, movement to another place, which is not implied in generation, and so generation is not simply procession, but a certain kind of procession. On the other hand, spiration, although it is a specific procession like generation, is nevertheless simply procession: for the proper act of a spirit and of love is to proceed. We willingly concede that procession from one or from two does not affect the notion of procession. As for the alleged similarity to generation, which is a transit from a male to a female, the example proves nothing and is very much out of place in a question where all is purity,....and surely it seems rash to think or believe that the spirating power of the Son related to the spirating power of the Father like the female and male faculties of generation. Hence the objection proves nothing.
Let us turn to our question: Is procession predicated equivocally or univocally? If the term is unqualified, it indicates local motion and voluntary motion. Wherefore even animals, when moved by appetite, are said, in De anima, III, to move with a processive motion. To proceed simply by such motion befits the Holy Spirit, because love and spirit proceed voluntarily, and, so to speak, processively. Thus understood, procession is not attributed to the Son except in a qualified sense. But if procession is taken in a sense similar to the process of an effect from a cause, as Dionysius says in the Liber de divinis nominibus, chap. 4, namely, that things which are multiple in their processes are one in their principle, then the term procession is employed in an extended sense, and befits both the generation of the Son and the spiration of the Holy Spirit; and so there is no reason why it should not befit the Son in one sense and the Holy Spirit in another, and why the sense in which it is said of a son in created nature, in which priority and posterity are possible, should not prevail over the sense in which it is said of the Holy Spirit; for procession by generation looks toward being, whereas procession by love in created nature looks only toward well-being. But all this is meaningless in God, in whom nothing is principal or secondary: just as the Son has His being from the Father by generation, so the Holy Spirit has His being from the Father and the Son by spiration. Therefore procession thus understood equally befits the Son and the Holy Spirit, but in different manners."
-St. Albert the Great