 It may seem odd to quote Aquinas at such length in an essay devoted to Aristotle’s categories, but I have done so for two reasons. First, as Ackrill’s and Furth’s comments illustrate, Aristotle’s scheme has been severely criticized by scholars and philosophers alike. Aquinas’s comments about quality, however, show that in the hands of a truly talented interpreter — and there certainly has been no interpreter of Aristotle greater than Aquinas — many of the criticisms can be met. Second, and more importantly, the attention that Aquinas gives to the category of quality is indicative of one of the most important facts about Aristotle’s categories, namely its profound historical importance in the development of metaphysical speculation. Whether philosophers have agreed or disagreed with Aristotle’s categorial scheme, his categorialism has played a significant instrumental role — it has provided in the millennia since its appearance the starting point for a great deal of metaphysical inquiry. In this respect, it can be compared to the quantifier in Twentieth-century metaphysics. Whether or not the quantifier is of philosophical interest, it is hard to imagine twentieth-century analytical metaphysics without it. So, to the extent that interest in the history of philosophy lies in the way in which ideas have had an influence from generation to generation, Aristotle’s categorial scheme is worth studying not only for the doctrines it contains but also for the interest that other philosophers have taken in it and the philosophy that they produced by using it as a springboard. (See the entry on  Commentators on Aristotle.)