Theorem pm4.67

Description: Theorem *4.67 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.67	$$⊢\neg \left(\neg \phi \to \neg \psi \right)\leftrightarrow \left(\neg \phi \wedge \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm4.63	$$⊢\neg \left(\neg \phi \to \neg \psi \right)\leftrightarrow \left(\neg \phi \wedge \psi \right)$$