Theorem pm4.64

Description: Theorem *4.64 of WhiteheadRussell p. 120. (Contributed by NM, 3-Jan-2005)

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

Proof

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