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Metamath Proof Explorer

Theorem pm2.52

Description: Theorem *2.52 of WhiteheadRussell p. 107. (Contributed by NM, 3-Jan-2005) (Proof shortened by Wolf Lammen, 8-Oct-2012)

		Ref	Expression
	Assertion	pm2.52	$⊢ \neg (φ \to ψ) \to (\neg φ \to \neg ψ)$

Step	Hyp	Ref	Expression
1		conax1k	$⊢ \neg (φ \to ψ) \to (\neg φ \to \neg ψ)$