Strangely enough, everybody seems to steer clear of defining "set". Fraenkel and Bar-Hillel deal mainly with antinomies and with innumerable axiomatic systems trying to avoid them in different ways, but don't define "set". The closest to a definition is Levy's following non-definition:

**By set we mean a completely structure-free set, and therefore a set is determined solely by its members**

We shall not dwell on this unfortunate formulation looking like a typo in otherwise well and clearly written book, but the fact remains - we still don't know what is "set, what is "member", nor what it means "to have members". The way out of the deadlock consists, as usually, in replacing the definition with a symbolically expressed axiom.

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