category an ultimate class. Categories are the highest genera of entities in the world. They may contain species but are not themselves species of any higher genera. Aristotle, the first philosopher to discuss categories systematically, listed ten, including substance, quality, quantity, relation, place, and time. If a set of categories is complete, then each entity in the world will belong to a category and no entity will belong to more than one category. A prominent example of a set of categories is Descartes’s dualistic classification of mind and matter. This example brings out clearly another feature of categories: an attribute that can belong to entities in one category cannot be an attribute of entities in any other category. Thus, entities in the category of matter have extension and color while no entity in the category of mind can have extension or color. See also ARISTOTLE , GENUS GENERALISSIMUM , RYL. J.W.M.