Theorem pm4.63

Description: Theorem *4.63 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

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

Proof

Step	Hyp	Ref	Expression
1		df-an	$$⊢\left(\phi \wedge \psi \right)\leftrightarrow \neg \left(\phi \to \neg \psi \right)$$
2	1	bicomi	$$⊢\neg \left(\phi \to \neg \psi \right)\leftrightarrow \left(\phi \wedge \psi \right)$$