4 ideas
9565 | Zermelo made 'set' and 'member' undefined axioms [Zermelo, by Chihara] |
3339 | For Zermelo's set theory the empty set is zero and the successor of each number is its unit set [Zermelo, by Blackburn] |
18946 | Unreflectively, we all assume there are nonexistents, and we can refer to them [Reimer] |
18758 | Validity is provable, but invalidity isn't, because the model is infinite [Church, by McGee] |