The goal here is to understand something of how St. Thomas thinks and talks about the Trinity, as well as to understand some of the implications his doctrine of the Trinity holds for how we might think about Creation at large. What I want to do here is to work through some of Thomas’s arguments, but what I’m ultimately interested in at present is less the particulars of his treatment than with the specific scholastic spirit with which he handles the doctrine of the Trinity. What I am also particularly interested in is how Thomas uses classical philosophical concepts to articulate Christian truth, but in doing so ends up radically transforming what these concepts mean, and so ends up raising his classical philosophical inheritance to previously unattained intellectual heights.
Of the seventeen questions contained in the Summa’s “Treatise on the Trinity,” I will be focusing only on the first three.
q. 27 “The Processions of the Divine Persons”
q. 28 “The Divine Relations”
q. 29 “The Divine Persons”
What I want to suggest is that you think of these as three distinct yet related “waves,” by means of which Thomas approaches the doctrine of the Trinity: there is a lot of overlap between each of these questions, but like successive waves in a rising tide, each question makes it a little further up the shore. (This isn’t a bad metaphor, incidentally, for thinking about the progression of the Summa as a whole.)
Furthermore, I want to suggest that there is a sort of intellectual “ascent” that St. Thomas’s discussion undergoes in these questions. Thomas begins with a discussion of the divine persons as “processions.” Although Thomas derives the idea of procession in the godhead from Scripture, the idea is also central to the whole philosophy of Neoplatonism, according to which the entire structure of reality is organized around a series of processions or “emanations” from a higher order of reality to a lower order of reality. One way of viewing question 27, accordingly, is to see St. Thomas as appropriating the Neoplatonic logic of emanation or procession in an attempt to understand or at least clarify the doctrine of the Trinity. (The concept of procession was also central to Bonaventure’s and the Franciscan tradition’s understanding of that which constituted the persons of the godhead as distinct persons. As we see here, the concept of procession is where St. Thomas begins his account of the persons of the Trinity, but as I am suggesting, he moves beyond this to what he and the Dominican tradition of Trinitarian theology more generally regard as the more central and determinative property distinguishing the persons of the Trinity as persons, namely the personal property of relation. Parallel to this difference, incidentally, is the fact that the Franciscan Bonaventure, for example, was far more Augustinian and Platonic in his thinking or sensibilities than the comparatively more Aristotelian Aquinas. Hence Bonaventure’s emphasis of procession or emanation over against Aquinas’s emphasis on the Aristotelian and Boethian concept of relation.)
Thus, while the concept of procession is a good place to start the discussion of the persons of the Trinity, Thomas does not want to leave us there. Why might that be? First, Thomas recognizes that the opportunities for misunderstanding the processions in God are abundant, and it may seem, moreover, that the idea of procession makes the persons of the godhead too independent of or extrinsic to each other. After all, as Thomas argues in his later “treatise on creation,” creation too is a “procession” or “emanation” from God, yet one that is entirely distinct from him. But surely this is not how we are to understand the processions of the divine persons. And so in the next question Thomas seeks to clarify that these processions from God are only so many relations within God. This brings Thomas to the next question, question 28, in which he tries Aristotle’s concept of relation on for size in an attempt to elucidate the Trinity.
But what might be the potential problem in thinking of the persons of the godhead in terms of relations? Just as the paradigm of emanation or procession, viewed from one perspective, threatened to make the persons of the godhead too independent of each other, without further qualification the concept of relation might seem to threaten to undermine their individuality and subsisting character. And so in the third question Thomas may be seen to push in the opposite direction: the processions which are also relations are also three distinct or individual persons. If procession is a characteristically Neoplatonic idea, and relation is an Aristotelian one, perhaps we might say that the concept of person is of more Hebraic or Christian extraction.
What we see in these three questions, accordingly, is Thomas drawing from different philosophical and theological traditions and sources in an attempt to render intelligible the Christian affirmation of the Father, Son, and Holy Ghost, who are nevertheless not three gods but one. We also see here a pattern that is very typical of how St. Thomas thinks and argues in general: his thought moves back and forth, back and forth, one article or question pushing in one direction, the next article or question balancing the discussion out by pushing in the other direction. This rhythm I like to refer to as “Aquinas’s pendulum”: tick-tock goes the Thomistic clock.
 The opposition between Aristotelian relation and Neoplatonic procession, it should be noted, can only be taken so far, for as Thomas Friedman observes, one way of viewing the whole difference between the Dominican from the Franciscan Trinitarian theologies is their choice to privilege one of Aristotle’s ten categories over another: where the Dominicans privileged relation, the Franciscans privileged act or action. Friedman, Medieval Trinitarian Thought from Aquinas to Ockham, 5.