Metamath Proof Explorer

Theorem pm2.13

Description: Theorem *2.13 of WhiteheadRussell p. 101. (Contributed by NM, 3-Jan-2005)

Ref Expression
Assertion pm2.13 
|- ( ph \/ -. -. -. ph )

Proof

Step Hyp Ref Expression
1 notnot 
 |-  ( -. ph -> -. -. -. ph )
2 1 orri 
 |-  ( ph \/ -. -. -. ph )