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Metamath Proof Explorer

Theorem pm4.8

Description: Theorem *4.8 of WhiteheadRussell p. 122. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm4.8 
|- ( ( ph -> -. ph ) <-> -. ph )

Proof

Step Hyp Ref Expression
1 pm2.01 
 |-  ( ( ph -> -. ph ) -> -. ph )
2 ax-1 
 |-  ( -. ph -> ( ph -> -. ph ) )
3 1 2 impbii 
 |-  ( ( ph -> -. ph ) <-> -. ph )