Not every concept can be defined in terms of other concepts, since such an approach would lead to circularity. Certain concepts must be given nonverbal, ostensive definitions. An ostensively defined concept is identified by a demonstrative gesture, e. g., by pointing to some of its referents in reality. ("Ostensive" derives from a root meaning "to display" and is etymologically related to "ostentatious.") Two categories of concepts are defined ostensively:

• Concepts of sensations, such as red (); and

• axiomatic concepts (e. g., existent, which clearly cannot be a sub-concept of a broader genus).

For a concept such as existent, the course Glossary provides explanatory remarks in lieu of a true definition.

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