Theorem wl-luk-notnotr

Description: Converse of double negation. Theorem *2.14 of WhiteheadRussell p. 102. In classical logic (our logic) this is always true. In intuitionistic logic this is not always true; in intuitionistic logic, when this is true for some ph , then ph is stable. Copy of notnotr with a different proof. (Contributed by Wolf Lammen, 17-Dec-2018) (New usage is discouraged.) (Proof modification is discouraged.)

	Ref	Expression
Assertion	wl-luk-notnotr	$$⊢\neg \neg \phi \to \phi$$

Proof

Step	Hyp	Ref	Expression
1		wl-luk-id	$$⊢\neg \phi \to \neg \phi$$
2	1	wl-luk-con1i	$$⊢\neg \neg \phi \to \phi$$