Theorem pm4.67

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

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

Proof

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