necessarily or demonstrably true; "demonstrable truths"
capable of being demonstrated or proved; "obvious lies"; "a demonstrable lack of concern for the general welfare"; "practical truth provable to all men"- Walter Bagehot
having a bearing on or connection with the subject at issue; "the scientist corresponds with colleagues in order to learn about matters relevant to her own research"
an intricate knot tied by Gordius, the king of Phrygia, and cut by the sword of Alexander the Great after he heard that whoever undid it would become ruler of Asia
any very difficult problem; insoluble in its own terms