Metamath Proof Explorer


Axiom ax-frege31

Description: ph cannot be denied and (at the same time ) -. -. ph affirmed. Duplex negatio affirmat. The denial of the denial is affirmation. Identical to notnotr . Axiom 31 of Frege1879 p. 44. (Contributed by RP, 24-Dec-2019) (New usage is discouraged.)

Ref Expression
Assertion ax-frege31 ( ¬ ¬ 𝜑𝜑 )

Detailed syntax breakdown

Step Hyp Ref Expression
0 wph 𝜑
1 0 wn ¬ 𝜑
2 1 wn ¬ ¬ 𝜑
3 2 0 wi ( ¬ ¬ 𝜑𝜑 )