Sentence operators and user operators are used in forming complex sentences. Definition operators are used in forming definitions. Object constants are used to denote individual objects. Function constants denote functions on those objects. Relation constants denote relations.
Logical constants express conditions about the world and are either true or false.
The differences between these categories of constants is entirely semantic. The legal expressions of KIF are formed from lexemes according to the rules presented in this section. Definitions and sentences are called forms . A knowledge base is a finite set of forms. Individual variables, constants, character references, strings, and blocks were discussed earlier.