Theorem notnotrALT

Description: Converse of double negation. Alternate proof of notnotr . This proof is notnotrALTVD automatically translated and minimized. (Contributed by Alan Sare, 21-Apr-2013) (Proof modification is discouraged.) (New usage is discouraged.)

	Ref	Expression
Assertion	notnotrALT	$$⊢\neg \neg \phi \to \phi$$

Proof

Step	Hyp	Ref	Expression
1		id	$$⊢\neg \neg \phi \to \neg \neg \phi$$
2		pm2.21	$$⊢\neg \neg \phi \to \left(\neg \phi \to \neg \neg \neg \phi \right)$$
3	1 2	mt4d	$$⊢\neg \neg \phi \to \phi$$