ABSTRACT
In the Propositional Calculus (PC) the authors consider solely complete statements or propositions and some of the ways in which these complete propositions can be combined or operated on to form complex propositions. The only variables they have used have been propositional variables, i.e. variables which take as their values whole propositions, whether simple or complex. This chapter presents a set of variables which can take as their values expressions. Such expressions are known in logic as predicates. Roughly, a predicate is any expression which stands for a property which a thing or person can possess. The word ‘property’ is being used in a very wide sense. By combining predicate variables and individual-variables the authors can construct certain special types of propositional schemata. The formation rules for predicate-expressions are exactly parallel to those for well-formed formula in PC.
