Category concepts are important for constructing the definitions with which we express the essential properties of things. Chatton’s particular interest are the real definitions of species concepts – “specific definitions” – and the things that those concepts signify, which are, to use modern terminology, natural kinds like human beings and roses. Specific definitions are constructed by compounding a genus concept and an essential difference concept. For example, the definition of human being is rational animal, where ‘rational’ is the essential difference and ‘animal’ is the genus; the definition of animal is sentient organism, where ‘sentient’ is the essential difference and ‘organism’ the genus. Two key semantic features, connected to definitions, were commonly ascribed to category concepts, which are (1) essential and (2) universal predicates. The terms ‘animal’ and ‘sentient’ are essentially predicable of each and every term falling under ‘animal,’ e.g., ‘cat,’ ‘human being,’ ‘frog,’ etc., precisely because ‘animal’ and ‘sentient’ appear in the definitions of ‘cat,’ etc.