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Actually, the thing that is excluded in constructive mathematics is ~ ~ P -> P. ~ ~ ~ P -> ~ P is a theorem also in constructive mathematics.
by cjfd 6y ago
Actually, the thing that is excluded in constructive mathematics is ~ ~ P -> P. ~ ~ ~ P -> ~ P is a theorem also in constructive mathematics.
- a1369209993 6y ago> ~ ~ ~ P -> ~ P is a theorem also in constructive mathematics. Could you provide a citation for this? I can vaguely see how that might work, being ~~Q->Q for the special case where Q can be decomposed into Q = ~P, but it's obviously rather difficult to search for.
- millimeterman 6y agoHere is a StackExchange post: https://math.stackexchange.com/questions/2453533/triple-negation-in-intuitionistic-logic https://math.stackexchange.com/questions/2453533/triple-nega...
- cjfd 6y agoIt is very easy to prove. Note first that ~P is an abbreviation for P -> False. So we can rewrite this as (~ ~ P -> False) -> P -> False. This is tautologically true by modes ponens if P -> ~ ~ P. But that is a well known fact.