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Theorem notnot1 114
Description: Converse of double negation. Theorem *2.12 of [WhiteheadRussell] p. 101. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 2-Mar-2013.)
Assertion
Ref Expression
notnot1 (φ → ¬ ¬ φ)

Proof of Theorem notnot1
StepHypRef Expression
1 id 19 . 2 φ → ¬ φ)
21con2i 112 1 (φ → ¬ ¬ φ)
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:  notnoti  115  con1d  116  con4i  122  notnot  282  biortn  395  pm2.13  407  eueq2  3010  ifnot  3700
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