Metamath Proof Explorer

Theorem frege42

Description: Not not id . Proposition 42 of Frege1879 p. 47. (Contributed by RP, 24-Dec-2019) (Proof modification is discouraged.)

Ref Expression
Assertion frege42 
|- -. -. ( ph -> ph )

Proof

Step Hyp Ref Expression
1 frege27 
 |-  ( ph -> ph )
2 ax-frege41 
 |-  ( ( ph -> ph ) -> -. -. ( ph -> ph ) )
3 1 2 ax-mp 
 |-  -. -. ( ph -> ph )