Theorem pm4.65

Description: Theorem *4.65 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.65	⊢ ( ¬ ( ¬ 𝜑 → 𝜓 ) ↔ ( ¬ 𝜑 ∧ ¬ 𝜓 ) )

Proof

Step	Hyp	Ref	Expression
1		pm4.61	⊢ ( ¬ ( ¬ 𝜑 → 𝜓 ) ↔ ( ¬ 𝜑 ∧ ¬ 𝜓 ) )