Theorem axfrege28 38123

Description: Contraposition. Identical to con3 149. Theorem *2.16 of [WhiteheadRussell] p. 103. (Contributed by RP, 24-Dec-2019.)

Assertion

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

Proof of Theorem axfrege28

Step	Hyp	Ref	Expression
1		con3 149	1 |- ( ( ph -> ps ) -> ( -. ps -> -. ph ) )

Syntax hints:   -. wn 3   -> wi 4

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8

This theorem is referenced by: (None)