Metamath Proof Explorer


Theorem axfrege31

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

Ref Expression
Assertion axfrege31 ¬ ¬ φ φ

Proof

Step Hyp Ref Expression
1 notnotr ¬ ¬ φ φ