Theorem axfrege31

Description: Identical to notnotr . Axiom 31 of Frege1879 p. 44. (Contributed by RP, 24-Dec-2019)

	Ref	Expression
Assertion	axfrege31	$$⊢\neg \neg \phi \to \phi$$

Proof

Step	Hyp	Ref	Expression
1		notnotr	$$⊢\neg \neg \phi \to \phi$$