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Theorem frege42 40478
Description: Not not id 22. Proposition 42 of [Frege1879] p. 47. (Contributed by RP, 24-Dec-2019.) (Proof modification is discouraged.)
Assertion
Ref Expression
frege42 ¬ ¬ (𝜑𝜑)

Proof of Theorem frege42
StepHypRef Expression
1 frege27 40443 . 2 (𝜑𝜑)
2 ax-frege41 40477 . 2 ((𝜑𝜑) → ¬ ¬ (𝜑𝜑))
31, 2ax-mp 5 1 ¬ ¬ (𝜑𝜑)
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4
This theorem was proved from axioms:  ax-mp 5  ax-frege1 40422  ax-frege8 40441  ax-frege41 40477
This theorem is referenced by:  frege43  40479
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