How did the modern equation of subjecthood and agency come to be? How did the notion of “I” or ego come to be substantialized? An extremely influential argument of Heidegger makes this an innovation of Descartes. Alain de Libera argues that this is too hasty, and that the groundwork for this identification was actually laid in the later middle ages. I’m continuing a high-level treatment of de Libera’s extremely important “archaeology of the subject“ (see also On a Philosophical Grammar).
Answering this question will involve an extended historical odyssey through complex interactions between Aristotelian and Augustinian views, and much more. De Libera sees Aquinas in his polemic against Averroes raising four interrelated questions of a more fundamental nature: Who thinks? What is the subject of thought? Who are we? What is man? The second of these seems to have been first asked by Averroes. The other three are largely attributable to Aquinas and his contemporaries, in their reactions to Averroes.
Several points of Aristotelian interpretation (What is substance? What is form? What is act? What is an efficient cause? What is the soul?) will be relevant to answering these, as will Augustine’s meditations on personhood and the nature of the Trinity. De Libera notes that John Locke — a major contributor to modern views on “the subject” — was deeply involved in debates on trinitarian theology. He also discusses Franz Brentano’s modern revival of the medieval notion of intentionality. The medieval version was closely bound up with a notion of “inexistence” or “existing in” of mental objects (forms separated from their matter) in the soul.
In the Categories, Aristotle gives substance the logical sense of something standing under something else. This influenced the Greek grammarians who formulated the notion of a grammatical subject. But in the Metaphysics, he treats this as only a starting point that is quickly superseded by an identification of substance with form or “what it was to have been” a thing, before moving into an account of substance as potentiality and actuality.
De Libera notes a historic division among readers of Aristotle’s treatise On the Soul between those who interpret the soul as an attribute of the body, and those who treat it as a substance in its own right. The latter position has different meanings, depending on whether substance is taken in the “standing under” sense or in the sense of form. De Libera will be particularly interested in the consequences of a further family of positions that make the non-obvious equation of human actions and passions with attributes of the soul.
He notes that “category” in Greek originally meant accusation, and relates this to Locke’s characterization of personhood as a “forensic” notion. We have here to do with subtle relations between attribution, inherence, and imputation with respect to actions and passions in relation to the soul. But what is an action? Must we explain an act in terms of a substantial subject’s power of efficient causation in a late scholastic sense that is far from Aristotle’s? (See also Expansive Agency; Brandomian Forgiveness.)