The present paper attempts to state and prove a completeness theorem for the system S5 of [1], supplemented by first-order quantifiers and the sign of equality. We assume that we possess a denumerably infinite list of individual variables a, b, c, …, x, y, z, …, xm, ym, zm, … as well as a denumerably infinite list of n-adic predicate variables Pn, Qn, Rn, …, Pmn, Qmn, Rmn,…; if n=0, an n-adic predicate variable is often called a “propositional variable.” A formula Pn(x1, …,xn) is an n-adic prime formula; often the superscript will be omitted if such an omission does not sacrifice clarity.

In [1], it is conjectured that if S is a sentence in the first-order functional calculus with identity, and every subsystem of every finite relational system which satisfies S also satisfies S, then S is finitely equivalent to a universal sentence. (Two sentences are finitely equivalent if and only if they are satisfied by the same finite relational systems.) The following sentence S refutes that conjecture, and moreover S is satisfied by all finite subsystems of all (finite or infinite) relational systems which satisfy it.1S contains as predicate letters only the two-place predicate letters ≦, R (and the identity symbol =).

By quite elementary means, one can find “large” examples difficult, if not (for practical purposes) impossible, to be managed by that host of methods, after Quine, for minimizing expressions in alternational normal form. Because the workability rather than existence of an algorithm for minimizing logical formulae is generally critical, it may be pertinent to outline briefly the derivation of these “large” examples. Some more general insight may also be gained about simplification techniques.

By intermediate prepositional logics we mean prepositional logics between the intuitionistic and classical logics.

K. Gödel [1] proved that there is a set of intermediate prepositional logics which possesses the order type ω. The method enables us to define intermediate logics in terms of axioms and rules of inference. We shall call it the axiomatic method.

According to Kleene [8] and Post [13] the definition of recursive relations (or sets) can be given most simply via the definition of recursively enumerable (r.e.) relations. The latter can be defined by finite systems of finite rules for deriving strings of symbols, each rule being of the form

each formula or being obtained by juxtaposition from atomic symbols and variables ranging over strings of atomic symbols.