Q. 28 “The Divine Relations”
As I mentioned earlier, however, one danger of emphasizing the status of the divine persons as divine processions is that, in doing so, one might overstress their distinctness at the expense of their unity. Thus, in the following question, question 28, we get what I am calling Thomas’s “second” wave in his explication of the doctrine of the Trinity, in which he moves to a consideration of the three persons as three distinct “relations.” And as I said earlier, here the inspiration is more directly Aristotelian (via Boethius’s De Trinitate), but similar to what we saw happen in the case of Neoplatonism in the previous question, here too Aristotle must undergo a transformation or, if you prefer, a conversion, before he can be made useful to Christ. Having explained the fact that there are processions within the godhead, the question remains for St. Thomas as to what precisely these things are which the processions have produced. The background to this discussion is Aristotle’s ten categories of being:
Beginning with Aristotle’s ten categories, therefore, the question, then, is this: granted that the three persons each share a common, divine substance, in which of these other categories (if any) does their distinction or difference lie? Thomas’s answer, going all the way back to Boethius, is that they represent distinct relations.
Now, from an Aristotelian standpoint this is all extraordinarily paradoxical (and indeed absurd), for it says that the terminus of the divine procession, or that which the procession produces, is nothing other than the processual relationship itself. So what’s going on here? Philipp Rosemann states the matter well:
While in creatures, relations inhere as accidents in some substantial subject…, in God, in whom there is nothing accidental, relations must be understood as being subsistent, and as coinciding with the divine essence… Furthermore, whereas in the creaturely realm relations are founded upon a link between one substantial being and another which is exterior to the first…, in God the two poles of a relation are not defined in reference to substances exterior to each other; rather, the opposite poles are constituted by the subsistent relation itself…. the Father is not a substance existing ‘before’ or apart from its relationship to the Son; the Father is fatherhood. Likewise, the Son is no being that could be conceived of independently of its relationship to the Father: the Son is sonship. ‘The Persons are the subsistent relations themselves’… At the summit of Being, substance and relation … lose their contradictory character.
Concluding overview: In using the classical, philosophical vocabulary and concepts of Aristotle and Neoplatonism to articulate the mysteries of the Christian faith, St. Thomas is plundering the Egyptians. But the result is the proverbial one of new wine and old wine skins.
Thomas writes that “the divine Persons are distinguished from each other according to the relations of origin…” The divine persons are identical in their substance, essence, or being. How then can they possibly be distinguished from one another? They differ from each other in their unique relations, specifically, in their unique “relations of origin.” The origins of this distinction is in Boethius’s treatise on the Trinity, in which he applies Aristotle’s categories to the question of the Trinity and answering the objection of how God’s being at once one and three is not a contradiction. The answer is that God is one in substance while three in his relations.
As it will turn out, the persons of the godhead not only differ in their relations of origin, but are in fact nothing other than their relations of origin. The Son, in other words, does not simply differ from the Father in that he is proceeds from or is generated by the Father, but he is nothing other than the very paternity (i.e, relation of paternity) of the Father. The Father is the Father by virtue of his paternal relationship to the Son, but the paternal relationship that differentiates the Father from the Son is nothing other than the Son himself: it is by the Son that the Father is differentiated from the Son. This makes the Son the difference of the Father: the Son is what makes the Father different. The Son is the Father’s paternity, making him the Father. It is therefore in and by the Son that the Father is the Father: the Father gets his fatherhood from the Son.
 Philipp W. Rosemann, Omne Agens Agit Sibi Simile
 For more on this, see Conor Cunningham, Genealogy of Nihilism, and “The Difference of Theology and Some Philosophies of Nothing,” Modern Theology 17, no. 3 (July 2001): 289-312.