William Craig’s scientific contributions

By a theory we shall always mean one of first order, having finitely many non-logical constants. Then for theories with identity (as a logical constant, the theory being closed under deduction in first-order logic with identity), and also likewise for theories without identity, one may distinguish the following three notions of axiomatizability. First, a theory may be recursively axiomatizable, or, as we shall say, simply, axiomatizable . Second, a theory may be finitely axiomatizable using additional predicates ( f. a. +), in the syntactical sense introduced by Kleene [9]. Finally, the italicized phrase may also be interpreted semantically. The resulting notion will be called s. f. a. +. It is closely related to the modeltheoretic notion PC introduced by Tarski [16], or rather, more strictly speaking, to PC ∩ AC δ . For arbitrary theories with or without identity, it is easily seen that s. f. a .+ implies f. a .+ and it is known that f. a. + implies axiomatizability. Thus it is natural to ask under what conditions the converse implications hold, since then the notions concerned coincide and one can pass from one to the other. Kleene [9] has shown: (1) For arbitrary theories without identity, axiomatizability implies f. a. +. It also follows from his work that : (2) For theories with identity which have only infinite models, axiomatizability implies f. a. +.