Theorem pm2.5

Description: Theorem *2.5 of WhiteheadRussell p. 107. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.5	$$⊢\neg \left(\phi \to \psi \right)\to \left(\neg \phi \to \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm2.5g	$$⊢\neg \left(\phi \to \psi \right)\to \left(\neg \phi \to \psi \right)$$