13 ideas
| 13201 | ∈ says the whole set is in the other; ⊆ says the members of the subset are in the other [Enderton] |
| 13206 | A 'linear or total ordering' must be transitive and satisfy trichotomy [Enderton] |
| 13204 | The 'ordered pair' <x,y> is defined to be {{x}, {x,y}} [Enderton] |
| 13200 | Note that {Φ} =/= Φ, because Φ ∈ {Φ} but Φ ∉ Φ [Enderton] |
| 13199 | The empty set may look pointless, but many sets can be constructed from it [Enderton] |
| 13203 | The singleton is defined using the pairing axiom (as {x,x}) [Enderton] |
| 13202 | Fraenkel added Replacement, to give a theory of ordinal numbers [Enderton] |
| 13205 | We can only define functions if Choice tells us which items are involved [Enderton] |
| 13163 | Circles must be bounded, so cannot be infinite [Leibniz] |
| 13162 | Sloth's Syllogism: either it can't happen, or it is inevitable without my effort [Leibniz] |
| 20930 | The existence of law is one thing, its merits and demerits another [Austin,J] |
| 19339 | Evil is a negation of good, which arises from non-being [Leibniz] |
| 13164 | God only made sin possible because a much greater good can be derived from it [Leibniz] |