 Over the past hundred years, skepticism about the possibility of offering a uniquely true and complete system of ontological categories has led discussion of categories to shift from attempts to offer complete systems of categories to attempts merely to draw particular distinctions, especially among our conceptual or linguistic categories. Work on category differences, unlike that on category systems, does not generally purport to answer deep metaphysical questions about what things or kinds of things exist; instead, category differences are typically articulated as a way of diagnosing and avoiding various philosophical problems and confusions. Nonetheless, even those who merely argue for category differences owe an account of the conditions under which two concepts, terms, or objects belong to different categories.