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

Proof

Step	Hyp	Ref	Expression
1		conax1k	$$⊢\neg \left(\phi \to \psi \right)\to \left(\neg \phi \to \neg \psi \right)$$