The Principle of Individuation according to St. Thomas Aquinas

Abstract

This work aims to initiate a comprehensive and definitive account of St. Thomas Aquinas' doctrine of the principle of the individuation of substances of a common species, which adds some sort of "quantity" or "dimensions" to the Aristotelian account of matter as the principle of individuation. After laying out the interpretative problem in its entirety through a review of the Scholastic and modern traditions of commentary, I determine the first step on the path to its solution, and take that first step by offering a properly limited interpretation of the account set forth in Question 4, article 2 of the Expositio super librum Boethii De trinitate. I argue that this text presents a sapiential metaphysical account of the principle of individuation informed by a properly metaphysical understanding which it leaves implicit. St. Thomas resolves the ratio of the numerically individual composite substance of a species as apprehended by the logician to its first per se principle, defined as "matter under dimensiones interminatae." As individuating, the dimensiones interminatae do not belong to the accidental category of quantity, but are merely a dimensional continuum, a certain composite of a potency--the parts of dimensions, which can be united or divided--and the unifying act of situs, the "order of the parts in the whole," or beginning-middle-end structure, by virtue of which the dimensions possess in themselves the ratio of the numerical individual. In each of these respects, the dimensions qualify the potency of the matter subject to them. Qua potency, the dimensiones interminatae qualify matter's intrinsic potency for unity with form in the substance as a whole by restricting its scope in the real order. Qua act, they qualify this complex restricted potency in a merely rational manner, rendering its restricting potency (i.e., that of the dimensional parts for situs) actual, and thus they make the complex restricted potency of matter intelligible, possessed of the ratio of the numerical individual. Accordingly, matter under dimensiones interminatae is this (and not that) matter, one unified principle belonging to the category of substance. In the properly metaphysical understanding of individuation which underlies the explicit account given in Question 4, article 2 of the Expositio, matter is understood as the potency for the corruptibly contingent mode of the act of substantial existence. Being subjected to the restricting potency of the dimensiones interminatae renders matter thus considered a principle of contingency, in the real order, in respect of divisibility. As before, this complex restricted potency is rendered partially actual in the rational order, and thus the ground of the ratio of the numerically individual substance qua being, by the dimensiones interminatae according to the act of situs. In this way, matter is constituted as this matter, this potency for the corruptibly contingent mode of existence, and not that matter--or in other words, it is constituted as numerically individual matter, the first per se principle of individuation.