Theorem notnotri

Description: Inference associated with notnotr . For a shorter proof using ax-2 , see notnotriALT . (Contributed by NM, 27-Feb-2008) (Proof shortened by Wolf Lammen, 15-Jul-2021) Remove dependency on ax-2 . (Revised by Steven Nguyen, 27-Dec-2022)

	Ref	Expression
Hypothesis	notnotri.1	$$⊢\neg \neg \phi$$
Assertion	notnotri	$$⊢\phi$$

Proof

Step	Hyp	Ref	Expression
1		notnotri.1	$$⊢\neg \neg \phi$$
2	1	pm2.21i	$$⊢\neg \phi \to \neg \neg \neg \phi$$
3	1 2	mt4	$$⊢\phi$$