It is the principle of unit-economy that necessitates the definition of
concepts in terms of essential characteristics. If, when in doubt, a man
recalls a concept’s definition, the essential characteristic(s) will give him
an instantaneous grasp of the concept’s meaning, i.e., of the nature of its
referents. For example, if he is considering some social theory and recalls
that “man is a rational animal,” he will evaluate the validity of the theory
accordingly; but if, instead, he recalls that “man is an animal possessing a
thumb,” his evaluation and conclusion will be quite different.