Theorem con4

Description: Alias for ax-3 to be used instead of it for labeling consistency. Its associated inference is con4i and its associated deduction is con4d . (Contributed by BJ, 24-Dec-2020)

	Ref	Expression
Assertion	con4	$$⊢\left(\neg \phi \to \neg \psi \right)\to \left(\psi \to \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		ax-3	$$⊢\left(\neg \phi \to \neg \psi \right)\to \left(\psi \to \phi \right)$$