Definition

A definition is a statement that fixes the meaning of a term by specifying what it is. Aristotle’s formula — genus plus differentia — remains the backbone: name the kind, then say what distinguishes this instance from other members of that kind. “A container is a thing that holds other things” gives the genus (thing) and the differentia (holds other things).

Definitions can be real (claiming to state what a thing actually is) or nominal (stipulating how a term will be used). Most working definitions in a library are nominal — they fix usage for the purpose of building further claims. A real definition asserts something about the world; a nominal definition asserts something about language.

A good definition is positive — it says what the thing is, not what it isn’t. It is specific enough to exclude what it should exclude and general enough to include what it should include. It does not use the term being defined. And it earns its precision: an overly narrow definition excludes real cases, an overly broad one fails to distinguish.

Every term in this library is, at bottom, a definition. The quality of the library’s knowledge depends on the quality of its definitions.