Theorem pm2.13

Description: Theorem *2.13 of WhiteheadRussell p. 101. (Contributed by NM, 3-Jan-2005)

	Ref	Expression
Assertion	pm2.13	$$⊢\left(\phi \vee \neg \neg \neg \phi \right)$$

Proof

Step	Hyp	Ref	Expression
1		notnot	$$⊢\neg \phi \to \neg \neg \neg \phi$$
2	1	orri	$$⊢\left(\phi \vee \neg \neg \neg \phi \right)$$