Simplicius reports that Xenocrates and Andronicus reproached Aristotle for positing an excessive number of categories, which can conveniently be reduced to two: τὰ καθ᾽αὑτά and τὰ πρός τι. Simplicius, followed by several modern commentators, interprets this move as being equivalent to a division into substance and accidents. I aim to show that, as far as Xenocrates is concerned, this interpretation is untenable and that the substance-accidents contrast cannot be equivalent to Xenocrates’ per se-relative one. Rather, Xenocrates aimed to stress the (...) primacy of Plato’s binary distinction of beings, as presented at Sophist 255c13–4, over Aristotle’s list of the categories. (shrink)