A system of categories is a complete list of highest kinds or genera. Traditionally, following Aristotle, these have been thought of as highest genera of entities (in the widest sense of the term), so that a system of categories undertaken in this realist spirit would ideally provide an inventory of everything there is, thus answering the most basic of metaphysical questions: “What is there?” Skepticism about the possibilities for discerning the different categories of ‘reality itself’ has led others to approach category systems not with the aim of cataloging the highest kinds in the world itself, but rather with the aim of elucidating the categories of our conceptual system. Thus Kant makes the shift to a conceptualist approach by drawing out the categories that are a priori necessary for any possible cognition of objects. Since such categories are guaranteed to apply to any possible object of cognition, they retain a certain sort of ontological import, although this application is limited to phenomena, not the thing in itself.
After Kant, it has been common to approach the project of categories in a neutral spirit that Brian Carr (1987, 7) calls “categorial descriptivism”, as describing the categorial structure that the world would have according to our thought, experience, or language, while refraining from making commitments about whether or not these categories are occupied. Edmund Husserl approaches categories in something like this way, since he begins by laying out categories of meanings, which may then be used to draw out ontological categories (categories of possible objects meant) as the correlates of the meaning categories, without concern for any empirical matter about whether or not there really are objects of the various ontological categories discerned.
A system of ontological categories drawn out in any of these modes has the potential for a great many uses in philosophy, but those who would offer such systems of categories also face a variety of difficulties. They must address the issue of what the proper methods are by means of which categories are to be distinguished, how many categories there are and what they are, whether or not there is a single summum genus subsuming all other categories, and whether we should distinguish a single system of categories or multiple dimensions of categories—issues on which there has been little agreement.
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 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.