Theorem pm4.66

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

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

Proof

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