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	$$⊢\neg \phi \to \left(\phi \to \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		id	$$⊢\neg \phi \to \neg \phi$$
2	1	pm2.21d	$$⊢\neg \phi \to \left(\phi \to \psi \right)$$