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Metamath Proof Explorer

Theorem pm2.21

Description: From a wff and its negation, anything follows. Theorem *2.21 of WhiteheadRussell p. 104. Also called the Duns Scotus law. Its commuted form is pm2.24 and its associated inference is pm2.21i . (Contributed by NM, 29-Dec-1992) (Proof shortened by Wolf Lammen, 14-Sep-2012)

		Ref	Expression
	Assertion	pm2.21	$⊢ \neg φ \to (φ \to ψ)$

Proof

Step	Hyp	Ref	Expression
1		id	$⊢ \neg φ \to \neg φ$
2	1	pm2.21d	$⊢ \neg φ \to (φ \to ψ)$