Theorem pm4.67 417

Description: Theorem *4.67 of [WhiteheadRussell] p. 120. (Contributed by NM, 3-Jan-2005.)

Assertion

Ref	Expression
pm4.67	⊢ (¬ (¬ φ → ¬ ψ) ↔ (¬ φ ∧ ψ))

Proof of Theorem pm4.67

Step	Hyp	Ref	Expression
1		pm4.63 410	1 ⊢ (¬ (¬ φ → ¬ ψ) ↔ (¬ φ ∧ ψ))

Syntax hints:  ¬ wn 3   → wi 4   ↔ wb 176   ∧ wa 358

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-an 360

This theorem is referenced by: (None)