http://en.wikipedia.org/wiki/Intuitionistic_logic
Unproved statements in Intuitionistic logic are not given an intermediate truth value (as is sometimes mistakenly asserted). Indeed, you can prove that it has no third truth value, a result dating back to Glivenko in 1928. Instead they remain of unknown truth value, until they are either proved or disproved. Statements are disproved by deducing a contradiction from them.
Unproved statements in Intuitionistic logic are not given an intermediate truth value (as is sometimes mistakenly asserted). Indeed, you can prove that it has no third truth value, a result dating back to Glivenko in 1928. Instead they remain of unknown truth value, until they are either proved or disproved. Statements are disproved by deducing a contradiction from them.