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Theorem notnot2 104
Description: Converse of double negation. Theorem *2.14 of [WhiteheadRussell] p. 102. (Contributed by NM, 5-Aug-1993.) (Proof shortened by David Harvey, 5-Sep-1999.) (Proof shortened by Josh Purinton, 29-Dec-2000.)
Assertion
Ref Expression
notnot2 (¬ ¬ φφ)

Proof of Theorem notnot2
StepHypRef Expression
1 pm2.21 100 . 2 (¬ ¬ φ → (¬ φφ))
21pm2.18d 103 1 (¬ ¬ φφ)
Colors of variables: wff setvar class
Syntax hints:  ¬ wn 3  wi 4
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8
This theorem is referenced by:  notnotrd  105  notnotri  106  con2d  107  con3d  125  notnot  282  condan  769  ecase3ad  911
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