Thomas Aquinas

“ Whatever proceeds by way of outward procession is necessarily distinct from the source whence it proceeds, whereas, whatever proceeds within by an intelligible procession is not necessarily distinct; indeed, the more perfectly it proceeds, the more closely it is one with the source whence it proceeds. For it is clear that the more a thing is understood, the more closely is the intellectual conception joined and united to the intelligent agent; since the intellect by the very act of understanding is made one with the object understood. ”

“ Hence, each procession in God takes its name from the proper notion of will and intellect; the name being imposed to signify what its nature really is; and so it is that the Person proceeding as love receives the divine nature, but is not said to be born.
Reply to Objection 2: Likeness belongs in a different way to the word and to love. It belongs to the word as being the likeness of the object understood, as the thing generated is the likeness of the generator; but it belongs to love, not as though love itself were a likeness, but because likeness is the principle of loving. ”

“ But if there is a being whose life does not proceed from potentiality to act, procession (if found in such a being) excludes entirely the first kind of generation; whereas it may have that kind of generation which belongs to living things. ”

“ For whenever we understand, by the very fact of understanding there proceeds something within us, which is a conception of the object understood, a conception issuing from our intellectual power and proceeding from our knowledge of that object. ”

“ Thus, as the divine processions must be denominated from certain actions; no other processions can be understood in God according to goodness and the like attributes except those of the Word and of love, according as God understands and loves His own essence, truth and goodness. ”

“ Not everything derived from another has existence in another subject; otherwise we could not say that the whole substance of created being comes from God, since there is no subject that could receive the whole substance. So, then, what is generated in God receives its existence from the generator, not as though that existence were received into matter or into a subject (which would conflict with the divine self-subsistence) ”

“ Having considered what belongs to the unity of the divine essence, it remains to treat of what belongs to the Trinity of the persons in God. And because the divine Persons are distinguished from each other according to the relations of origin, the order of the doctrine leads us to consider firstly, the question of origin or procession ”

“ So what proceeds in God by way of love, does not proceed as begotten, or as son, but proceeds rather as spirit; which name expresses a certain vital movement and impulse, accordingly as anyone is described as moved or impelled by love to perform an action. ”