 While those who only make use of the idea of category differences (rather than purporting to offer a category system) have no need to worry about how to provide an exhaustive list of categories, they nonetheless owe an account of the conditions under which we can legitimately claim that two entities, concepts, or terms are of different categories, so that we know when a category mistake is (and is not) being made. Otherwise, they would face the charge of arbitrariness or ad hocery in views about which categories there are or where category differences lie. Yet there is little more agreement about the proper criteria for distinguishing categories than there is about what categories there are.