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Predicates are represented in CycL by constants or NARTs that are sometimes referred to as “CycL predicates” or, more casually, as “predicates.” Like CycL functions (the other kind of relation-denoting constants), CycL predicates can be used as the leading term (after the initial parenthesis) in CycL expressions. When a CycL predicate is applied to the right number and type of arguments, the expression formed is a CycL formula–a formula expressing a proposition about something. In contrast, expressions formed with functions as arg 0 (in the leading position) are terms and so do not express propositions.

By convention, constants that denote predicates begin with lowercase letters.