According to Alain de Libera in the second half of Archéologie du sujet vol. 1, Thomas Aquinas was instrumental in developing a view of the soul that was neither Aristotelian nor Augustinian, and that paved the way for the modern concept of “the subject” as an agent, long before Descartes. De Libera says that Aquinas did this in part by introducing the different, very abstract Aristotelian notion of subject (hypokeimenon, “thing underlying”, with no connotations of mind or agency) into the Augustinian model of the soul as an image of the Christian Trinity, and simultaneously introducing the Augustinian biblical Word into an Aristotelian model of abstractive knowledge. Aquinas also drew indirectly on Plotinus, and directly on his teacher Albert the Great’s use of pseudo-Dionysius. In doing so, he effectively removed the stigma Augustine had placed on treating the human soul as a “subject”.
Aristotle had suggested that there is a kind of identity between thinking and what it thinks. It is perhaps not accidental that we use different senses of the same English word “thought” for both. These should not be equated with subject and object in the modern sense; they both occupy parts of a kind of middle ground between what we call subject and object.
According to de Libera, Plotinus developed a kind of identity between three terms (nous, noeisis, noeton — intellect, intellection, intelligible object). His intellect and intelligible object are already somewhat closer to what we call subject and object. In between, he placed an act of thinking or intellection that was to have a kind of identity with both the intellect and the intelligible object.
Plotinus’ notion of act is also quite different from that of Aristotle. Aristotle calls the first principle a kind of pure act that is not an action in the ordinary sense, and has nothing else behind it; for Plotinus, the first principle is a power, and every act is the act of a power. For Aristotle, the first principle is also an end only; for Plotinus, it is both the end and the origin of all things.
The persons of the Trinity are supposed to have a sort of mutual immanence to one another that is completely unlike the case of something underlying something else. De Libera notes that Plotinus and his student Porphyry already used a similar concept of mutual immanence in their discussions of intellect. Augustine ranked his reading of Plotinus as a formative experience second only to his conversion to Christianity.
From the Christian neoplatonist pseudo-Dionysius, Albert the Great drew the notion of a “whole of powers” that is different from either a universal whole or an integral whole.
De Libera notes that the classic formula of the Trinity in Greek — one ousia, three hypostases — was confusingly translated into Latin as “one essence, three substances” or as “one substance, three persons”. By substitution, the coexistence of these two translations yields the obviously self-contradictory formula, “one substance, three substances”, which graphically illustrates the equivocation in medieval usages of “substance”.
(In deference to common usage, I have continued to use “substance” for Aristotle’s ousia, even though I think it is a terrible translation. “Essence” is better, provided we recognize that Plato and Aristotle had views of essence that were not “essentialist” in the sense of treating essences of things as pre-given or as something to take for granted.)
De Libera speaks of the need to parenthesize modern notions of subject and object in order to understand Augustine’s opposition to treating actions and passions of the soul as attributes of a substance. Conversely, for better or worse, Aquinas’ legitimation of this way of viewing the soul brings us closer to modern views. (I think Aristotle would have shared Augustine’s opposition to this formulation, but for different reasons. I think Aristotle regarded the whole human being — and not the soul or the body taken separately — as a “substance”.)
Aquinas introduced emphasis on both what de Libera calls an Aristotelian structure of subject-powers-activities and a pseudo-Dionysian structure of essence-power-operation into a Latin-speaking theological context that had been mainly dominated by Augustine. What I would call this double infusion of additional neoplatonic elements is said by some to have resulted in a more dynamic and relational way of viewing things. (In agreement with Gwenaëlle Aubry, however, I think Aristotelian potentiality is very different from neoplatonic power, even though they use the same Greek word.) Combined with Aquinas’ serious embrace of a version of Aristotelian hylomorphism, this infusion led to a simultaneously more positive and more dynamic view of worldly existence than had been common in the Augustinian tradition, which also helped lay the seeds of modernity.
A broadly neoplatonic view of the world in terms of powers and operations-of-powers thus turns out to have been very important for the emergence of the modern subject-as-agent (as well as, I would argue, the rise of the specific modern notion of causality). De Libera notes that Heidegger ignored both neoplatonism and theology in his famous account of the rise of the modern subject. Meanwhile, Aquinas’ legitimation of the treatment of actions and passions as attributes of a soul-subject-substance — coupled with the interweaving of such attribution with imputations of responsibility — seems to have contributed to a stronger notion of a self as something with univocal identity and sharp edges.