| 13824 | Proof theory began with Frege's definition of derivability [Frege, by Prawitz] |
| 15089 | Logical proof just explicates complicated tautologies [Wittgenstein] |
| 13801 | An 'informal proof' is in no particular system, and uses obvious steps and some ordinary English [Bostock] |
| 15406 | 'Induction' and 'recursion' on complexity prove by connecting a formula to its atomic components [Burgess] |