Theorem pm4.63

Description: Theorem *4.63 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.63	⊢ ( ¬ ( 𝜑 → ¬ 𝜓 ) ↔ ( 𝜑 ∧ 𝜓 ) )

Proof

Step	Hyp	Ref	Expression
1		df-an	⊢ ( ( 𝜑 ∧ 𝜓 ) ↔ ¬ ( 𝜑 → ¬ 𝜓 ) )
2	1	bicomi	⊢ ( ¬ ( 𝜑 → ¬ 𝜓 ) ↔ ( 𝜑 ∧ 𝜓 ) )