  Zermelo's Axiom of Separation is: Exist x: Forall y: (y in x <==> y in a &
E(y)) with definite(E) and parameter a. Thoralf Skolem suggested to
characterize the terminus "definite" by "the property E should be representable
by a FOL formula". But that is trivial. "definite" must mean more. The author
claims that "definite" means "in accordance with the theory of definitions of
logic". In this case the theorem of Cantor is no longer a theorem, but a
undecidable sentence, and has to be established explicitly as axiom. This is
not done by the community, but it is made a silent assumption that we can drop
the appendix "definite(E)" from the axiom of separation at all. But this is a
convention (even when it is silent) and it is nothing else than an axiom.

Logic