Theorem con3ALT2

Description: Contraposition. Alternate proof of con3 . This proof is con3ALTVD automatically translated and minimized. (Contributed by Alan Sare, 21-Apr-2013) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Assertion	con3ALT2	$$⊢\left(\phi \to \psi \right)\to \left(\neg \psi \to \neg \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		notnotr	$$⊢\neg \neg \phi \to \phi$$
2	1	imim1i	$$⊢\left(\phi \to \psi \right)\to \left(\neg \neg \phi \to \psi \right)$$
3	2	con1d	$$⊢\left(\phi \to \psi \right)\to \left(\neg \psi \to \neg \phi \right)$$