Theorem pm4.65

Description: Theorem *4.65 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.65	$$⊢\neg \left(\neg \phi \to \psi \right)\leftrightarrow \left(\neg \phi \wedge \neg \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		pm4.61	$$⊢\neg \left(\neg \phi \to \psi \right)\leftrightarrow \left(\neg \phi \wedge \neg \psi \right)$$