What model companionship can say about the Continuum problem

Abstract

We present recent results on the model companions of set theory, placing them in the context of the current debate in the philosophy of mathematics. We start by describing the dependence of the notion of model companionship on the signature, and then we analyze this dependence in the specific case of set theory. We argue that the most natural model companions of set theory describe (as the signature in which we axiomatize set theory varies) theories of H κ + , as κ ranges among the infinite cardinals. We also single out 2 ℵ 0 = ℵ2 as the unique solution of the Continuum problem which can (and does) belong to some model companion of set theory (enriched with large cardinal axioms). Finally this model-theoretic approach to set-theoretic validities is explained and justified in terms of a form of maximality inspired by Hilbert's axiom of completeness. notion of model companionship: that of absolute model companionship (AMC), see Section 5. AMC seems to be the correct notion of model companionship to apply to set theory. For example it is unknown to the authors whether there can be a signature for which the axiomatization of set theory in it admits a model companion which is not its AMC.