Description: We cannot (at the same time) affirm ph and deny ph . Identical to id . Proposition 27 of Frege1879 p. 43. (Contributed by RP, 24-Dec-2019) (Proof modification is discouraged.)
| Ref | Expression | ||
|---|---|---|---|
| Assertion | frege27 | |- ( ph -> ph ) |
| Step | Hyp | Ref | Expression |
|---|---|---|---|
| 1 | ax-frege1 | |- ( ph -> ( ps -> ph ) ) |
|
| 2 | frege26 | |- ( ( ph -> ( ps -> ph ) ) -> ( ph -> ph ) ) |
|
| 3 | 1 2 | ax-mp | |- ( ph -> ph ) |