glivenko instead remain unknown truth value either proved disproved statements disproved deducing contradiction consequence point view intuitionistic logic interpretation two valued logic even finite valued logic familiar sense although intuitionistic logic retains trivial propositions math top bot math classical logic ''proof'' propositional formula considered valid propositional value thus heyting 's notion propositions sets propositional formulae potentially non finite sets proofs heyting algebra semantics classical logic
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