[FOM] Some informative questions about intuitionistic logic and mathematics

Thomas Forster T.Forster at dpmms.cam.ac.uk
Mon Nov 7 09:45:18 EST 2005

On the subject of intuitionists not denying bivalence (Thank you, 
Richard, for that interesting posting) it may be worth noting that 
constructive propositional logic explicitly denies that there are 
more than two truth values.  The sequent

-(A <--> B), -(B <--> C), -(C <--> A) |-

(which says roughly that A, B and C cannot all have different truth-values)

is constructively correct..... but hard! It's guaranteed to 
keep even quite good students out of mischief for several hours

  URL: www.dpmms.cam.ac.uk/~tf   Tel: +44-1223-337981
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