The author notices ambiguity in Aquinas’ principle of individuation, or how it is that a universal substantial form becomes instantiated in primary substances yet remains universal (to the class). The paper’s first section examines the general notion of the principle of individuation, as well as the three components of primary substance in Aquinas’ and Aristotle’s philosophy. The second investigates Aquinas’ three-tiered concept of matter and how matter is said to be the principle of individuation. The third section clarifies Aquinas’s claim, pointing toward designated matter as signified by quantity as being the specific principle of individuation. But there’s a problem. It would seem that Aquinas is in fact arguing that a primary substance is the principle of individuation for a primary substance. The author provides a nine-point summary of this argument before anticipating a potential response from Aquinas that suggests that it is quantity (an accident! and not matter) which is the principle of individuation. But this is a troubling discovery, no?

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