Editing Predicate variable (section)

== Notation ==
Predicate variables should be distinguished from predicate constants, which could be represented either with a different (exclusive) set of predicate letters, or by their own symbols which really do have their own specific meaning in their [[domain of discourse]]: e.g. <math> =, \ \in , \ \le,\ <, \ \sub,... </math>.

If letters are used for both predicate constants and predicate variables, then there must be a way of distinguishing between them. One possibility is to use letters ''W'', ''X'', ''Y'', ''Z'' to represent predicate variables and letters ''A'', ''B'', ''C'',..., ''U'', ''V'' to represent predicate constants. If these letters are not enough, then numerical subscripts can be appended after the letter in question (as in ''X''<sub>1</sub>, ''X''<sub>2</sub>, ''X''<sub>3</sub>). 

Another option is to use Greek lower-case letters to represent such metavariable predicates. Then, such letters could be used to represent entire well-formed formulae (wff) of the predicate calculus: any free variable terms of the wff could be incorporated as terms of the Greek-letter predicate. This is the first step towards creating a higher-order logic.