Theorem pm4.66 409

Description: Theorem *4.66 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)

Assertion

Ref	Expression
pm4.66	⊢ ((¬ φ → ¬ ψ) ↔ (φ ∨ ¬ ψ))

Proof of Theorem pm4.66

Step	Hyp	Ref	Expression
1		pm4.64 361	1 ⊢ ((¬ φ → ¬ ψ) ↔ (φ ∨ ¬ ψ))

Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 176   ∨ wo 357

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

This theorem depends on definitions:  df-bi 177  df-or 359

This theorem is referenced by:  pm4.54  479