 Plainly, the enterprise of categorialism inaugurated by Aristotle runs deep in the philosophical psyche. Even so, despite its wide-reaching influence — and, indeed owing to that influence — any attempt to describe categorialism faces a significant difficulty: experts disagree on many of its most important and fundamental aspects. Each of the following questions has received markedly different answers from highly respected scholars and philosophers. What do the categories classify? What theory of predication underlies Aristotle’s scheme? What is the relationship between categorialism and hylemorphism, Aristotle’s other major ontological theory? Where does matter fit, if at all, in the categorial scheme? When did Aristotle write the Categories? Did Aristotle write the Categories? Is the list of kinds in the Categories Aristotle’s considered list, or does he modify his views elsewhere? Is Aristotle’s view of substance in the Categories consistent with his view of substance in the Metaphysics? Is there some method that Aristotle used in order to generate his list of categories? Is Aristotle’s categorialism philosophically defensible in whole or in part? If only in part, which part of categorialism is philosophically defensible?