Theorem pm4.57

Description: Theorem *4.57 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm4.57	$$⊢\neg \left(\neg \phi \wedge \neg \psi \right)\leftrightarrow \left(\phi \vee \psi \right)$$

Proof

Step	Hyp	Ref	Expression
1		oran	$$⊢\left(\phi \vee \psi \right)\leftrightarrow \neg \left(\neg \phi \wedge \neg \psi \right)$$
2	1	bicomi	$$⊢\neg \left(\neg \phi \wedge \neg \psi \right)\leftrightarrow \left(\phi \vee \psi \right)$$