In my lecture notes for Discrete Structures, the professor introduced a definition on functors in the "Syntax of Predicate Logic" section.
Definition of functors: Let us consider a collection of symbols called functors (each functor is associated to a natural number n, called its valence or arity, we say that the functor is n-ary). The 0-ary functors are called constants. Let us consider a collection of symbols called variables. Let us consider the two parenthesis symbols ( and ) and the comma symbol ,.
Can anyone tell me what is the intrinsic difference between functors and predicates?