Theorem pm4.66

Description: Theorem *4.66 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.66	$$⊢\left(\neg \phi \to \neg \psi \right)\leftrightarrow \left(\phi \vee \neg \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm4.64	$$⊢\left(\neg \phi \to \neg \psi \right)\leftrightarrow \left(\phi \vee \neg \psi \right)$$