Theorem axfrege28

Description: Contraposition. Identical to con3 . Theorem *2.16 of WhiteheadRussell p. 103. (Contributed by RP, 24-Dec-2019)

	Ref	Expression
Assertion	axfrege28	$$⊢\left(\phi \to \psi \right)\to \left(\neg \psi \to \neg \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		con3	$$⊢\left(\phi \to \psi \right)\to \left(\neg \psi \to \neg \phi \right)$$