Theorem pm4.81 355

Description: Theorem *4.81 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)

Assertion

Ref	Expression
pm4.81	|- ((-. ph -> ph) <-> ph)

Proof of Theorem pm4.81

Step	Hyp	Ref	Expression
1		pm2.18 102	. 2 |- ((-. ph -> ph) -> ph)
2		pm2.24 101	. 2 |- (ph -> (-. ph -> ph))
3	1, 2	impbii 180	1 |- ((-. ph -> ph) <-> ph)

Syntax hints:  -. wn 3   -> wi 4   <-> wb 176

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

This theorem depends on definitions:  df-bi 177

This theorem is referenced by: (None)