Modern commentators recognize the irony of Aristotle’s Categories becoming a central text for Platonic schools. For similar reasons, these commentators would perhaps be surprised to see Aquinas’s In VII Metaphysics, where he apparently identifies the secondary substance of Aristotle’s Categories with a false Platonic sense of “substance” as if, for Aristotle, only Platonists would say secondary substances are substances. This passage in Aquinas’s commentary has led Mgr. Wippel to claim that, for Aquinas, secondary substance and essence are not the same (...) thing and that Aristotle’s notion of essence is absent from the Categories. This paper—by closely analyzing the apparently contradictory divisions of “substance” in Aquinas’s In V and VII Metaphysics—shows that essence and secondary substance are not altogether distinct for Aquinas. Moreover, when the Categories is viewed by Aquinas as a work of logic, it is found largely to cut across the disputes between Platonism and Aristotelianism. (shrink)