  The second group is roughly intensional; they concern what categories are and the properties they have. The intension of a term is the set of properties that apply to the things of which the term is truthfully predicated and which are listed in its definition or taken to be implied by it. Thus the intension of ‘human’ is, say, rational animal, and includes living and corporeal. Again, philosophers disagree as to whether categories can be defined, and if they can be, how they are to be defined. In general, medieval authors reject the possibility of defining them for various reasons.  One reason is that most of these authors conceive categories as divisions of being, and being for them is not a genus. Since a definition requires a genus (“animal” in the definition of “human” given earlier), the categories cannot be defined. Another reason is that a definition requires a difference that distinguishes what is being defined from other types of thing within the genus (“rationality” for humans, within the genus “animal”), but the categories are the highest kind of things, so there is nothing outside of them that can be used to distinguish them.