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Metamath Proof Explorer

Theorem pm4.8

Description: Theorem *4.8 of WhiteheadRussell p. 122. (Contributed by NM, 3-Jan-2005)

		Ref	Expression
	Assertion	pm4.8	$⊢ (φ \to \neg φ) \leftrightarrow \neg φ$

Step	Hyp	Ref	Expression
1		pm2.01	$⊢ (φ \to \neg φ) \to \neg φ$
2		ax-1	$⊢ \neg φ \to (φ \to \neg φ)$
3	1 2	impbii	$⊢ (φ \to \neg φ) \leftrightarrow \neg φ$