Theorem con2

Description: Contraposition. Theorem *2.03 of WhiteheadRussell p. 100. (Contributed by NM, 29-Dec-1992) (Proof shortened by Wolf Lammen, 12-Feb-2013)

	Ref	Expression
Assertion	con2	$$⊢\left(\phi \to \neg \psi \right)\to \left(\psi \to \neg \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		id	$$⊢\left(\phi \to \neg \psi \right)\to \left(\phi \to \neg \psi \right)$$
2	1	con2d	$$⊢\left(\phi \to \neg \psi \right)\to \left(\psi \to \neg \phi \right)$$