predicate variables are also allowed to be bound to predicate letters which are unary or have higher arity, and when such letters represent propositional functions, such that the domain of the arguments is mapped to a range of different propositions, and when such variables can be bound by quantifiers to such sets of propositions, then the result is a higher-order predicate calculus, or higher-order logic. See also References Bibliography Rudolf Carnap and William H. Meyer. Introduction to Symbolic Logic and Its Applications. Dover Publications (June 1, 1958). Predicate logic Logic symbols