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Theorem frege42 41178
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 41143 . 2 (𝜑𝜑)
2 ax-frege41 41177 . 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 41122  ax-frege8 41141  ax-frege41 41177
This theorem is referenced by:  frege43  41179
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