Theorem pm4.8 354

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

Assertion

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

Proof of Theorem pm4.8

Step	Hyp	Ref	Expression
1		pm2.01 160	. 2 |- ((ph -> -. ph) -> -. ph)
2		ax-1 6	. 2 |- (-. ph -> (ph -> -. ph))
3	1, 2	impbii 180	1 |- ((ph -> -. ph) <-> -. ph)

This proof depends on syntax axioms:  -. wn 3   -> wi 4   <-> wb 176

This proof depends on axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8

This proof depends on definitions:  df-bi 177

This theorem is used by: (None)