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	⊢ ( ¬ ¬ 𝜑 → 𝜑 )

Proof

Step	Hyp	Ref	Expression
1		id	⊢ ( ¬ ¬ 𝜑 → ¬ ¬ 𝜑 )
2		pm2.21	⊢ ( ¬ ¬ 𝜑 → ( ¬ 𝜑 → ¬ ¬ ¬ 𝜑 ) )
3	1 2	mt4d	⊢ ( ¬ ¬ 𝜑 → 𝜑 )