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Metamath Proof Explorer

Theorem pm4.66

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

		Ref	Expression
	Assertion	pm4.66	$⊢ (\neg φ \to \neg ψ) \leftrightarrow (φ \lor \neg ψ)$

Step	Hyp	Ref	Expression
1		pm4.64	$⊢ (\neg φ \to \neg ψ) \leftrightarrow (φ \lor \neg ψ)$