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Qiaochu_Yuan comments on Second-Order Logic: The Controversy - Less Wrong
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ZF can't prove that models of ZF exist because proving models of ZF exist is equivalent to proving that ZF is consistent, and ZF can't prove its own consistency (if it is in fact consistent) by the incompleteness theorem. I don't think ZFC can prove the consistency of ZF either, but I'm not a set theorist.
Also not a set theorist, but I'm pretty sure this is correct. ZF+Con(ZF) proves Con(ZFC) (see http://en.wikipedia.org/wiki/Constructible_universe), so if ZFC could prove Con(ZF) then it would also prove Con(ZFC).