Metamath Proof Explorer

Theorem axfrege28

Description: Contraposition. Identical to con3 . Theorem *2.16 of WhiteheadRussell p. 103. (Contributed by RP, 24-Dec-2019)

Ref Expression
Assertion axfrege28 
|- ( ( ph -> ps ) -> ( -. ps -> -. ph ) )

Proof

Step Hyp Ref Expression
1 con3 
 |-  ( ( ph -> ps ) -> ( -. ps -> -. ph ) )