Many people imagine that the meaning of a concept is determined by its definition. If valid concepts are determined by the objective properties of their constituent units, however, then the relationship must be rather the opposite: the proper form of a definition is determined by the concept's meaning.

The purpose of definitions is to distinguish our concepts from one another and to identify which units are subsumed under a given concept. In its standard form, a verbal definition specifies a genus and one or more differentiae:

CONCEPT DIFFERENTIA GENUS

A human is a rational animal.

The genus—in this case, "animal"—is the broader concept of which the current concept is a sub-concept. By specifying the genus, we indicate how the concept fits into the conceptual hierarchy. The differentiae—in this case, just one differentia "rational"—specify how the concept's units differ in kind from others of the same genus.

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