  This, then, provides the content of Skolem's claim that axiomatizing set theory leads to a relativity of set-theoretic notions. It is important, here, to distinguish this claim from a more trivial claim which Skolem might be thought to be making. From one perspective, the algebraic conception of axiomatization leads to an obvious form of relativity: the elements which count as sets in one model may not count as sets in another model, the membership relation of one model may be different than the membership relation of another model, and this latter difference in membership relations may hold even if the two models happen to share the same domain. On this trivial notion of relativity, therefore, almost everything turns out to be relative, even simple notions like “x is the empty set” or “x is a singleton.” After all, an object could be a “singleton” in one model while being a “doubleton” in another model, or it could be “the empty set” in one model while being omitted entirely from another model's domain.