5 ideas
| 19389 | Truth is a characteristic of possible thoughts [Leibniz] |
| 19388 | True and false seem to pertain to thoughts, yet unthought propositions seem to be true or false [Leibniz] |
| 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] |
| 15034 | Are genera and species real or conceptual? bodies or incorporeal? in sensibles or separate from them? [Porphyry] |