17833 | The first-order ZF axiomatisation is highly non-categorical |

17832 | Zermelo showed that the ZF axioms in 1930 were non-categorical |

17834 | Non-categoricity reveals a sort of incompleteness, with sets existing that the axioms don't reveal |

17835 | Gödel show that the incompleteness of set theory was a necessity |

17837 | Zermelo allows ur-elements, to enable the widespread application of set-theory |

17836 | The General Continuum Hypothesis and its negation are both consistent with ZF |