In In V Metaphysics, lec. 9, Aquinas distinguishes between “being by accident” (ens per accidens) and “being by itself” (ens per se) and includes the nine accidental categories under the latter. But isn’t substance a being per se while accidents are, by definition, accidental beings? Several authors—including Ralph McInerny, Paul Symington, and Greg Doolan—have offered explanations of this strange classification. Drawing on an overlooked parallel text in the Posterior Analytics commentary and on Aquinas’s critique of Avicenna’s understanding of accidental denominatives, (...) this paper presents an alternative explanation of the lecture. In the process, it clarifies how Aquinas views the relation of the ten categories to predication and suggests important implications for how we should understand the analogy of being and the phrase “substantial being” (esse substantiale). (shrink)