 Aristotle’s Categories is a singularly important work of philosophy. It not only presents the backbone of Aristotle’s own philosophical theorizing but has exerted an unparalleled influence on the systems of many of the greatest philosophers in the western tradition. The set of doctrines in the Categories, which I will henceforth call categorialism, provides the framework of inquiry for a wide variety of Aristotle’s philosophical investigations, ranging from his discussions of time and change in the Physics to the science of being qua being in the Metaphysics, and even extending to his rejection of Platonic ethics in the Nicomachean Ethics. Looking beyond his own works, Aristotle’s categorialism has engaged the attention of such diverse philosophers as Plotinus, Porphyry, Aquinas, Descartes, Spinoza, Leibniz, Locke, Berkeley, Hume, Kant, Hegel, Brentano, and Heidegger (to mention just a few), who have variously embraced, defended, modified or rejected its central contentions. All, in their different ways, have thought it necessary to come to terms with features of Aristotle’s categorial scheme.