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Theorem notnot2 112
Description: Converse of double negation. Theorem *2.14 of [WhiteheadRussell] p. 102. This was the fifth axiom of Frege, specifically Proposition 31 of [Frege1879] p. 44. In classical logic (our logic) this is always true. In intuitionistic logic this is not always true; in intuitionistic logic, when this is true for some 
ph, then  ph is stable. (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  |-  ( -. 
-.  ph  ->  ph )

Proof of Theorem notnot2
StepHypRef Expression
1 pm2.21 108 . 2  |-  ( -. 
-.  ph  ->  ( -. 
ph  ->  ph ) )
21pm2.18d 111 1  |-  ( -. 
-.  ph  ->  ph )
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  113  notnotri  114  con2d  115  con3d  133  notnot  291  ecase3ad  929  notornotel1  28739  con3ALT  30933  zfregs2VD  31276  con3ALTVD  31351  notnot2ALT2  31362
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