0 ▁pages , ▁mainly ▁in ▁order ▁to ▁set ▁out ▁clearly ▁a ▁fundamental ▁reform ▁program ▁for ▁gover ning ▁the ▁country ▁correctly ▁according ▁to ▁Conf u cian ▁ide als . ▁During ▁the ▁years ▁of ▁ex ile ▁he ▁concentr ated ▁first ▁on ▁the ▁Book ▁of ▁Ch anges ▁( Y i ▁Ch ing ), ▁writing ▁in ▁ 1 8 0 5 ▁the ▁Ch u ye oks aje on . ▁A ▁reflection ▁on ▁the ▁Classic ▁of ▁Po etry ▁followed ▁in ▁ 1 8 0 9 . ▁He ▁wrote ▁on ▁politics , ▁eth ics , ▁economy , ▁natural ▁sciences , ▁medicine ▁and ▁music . ▁After ▁his ▁return ▁from ▁ex ile , ▁Das an ▁published ▁his ▁most ▁important ▁works : ▁on ▁juris pr ud ence ▁He um he um sin se o ▁( 1 8 1 9 ); ▁on ▁lingu istics ▁A e ong ak bi ▁( 1 8 1 9 ); ▁on ▁diplom acy ▁S ade k ory es an bo ▁( 1 8 2 0 ); ▁on ▁the ▁art ▁of ▁gover ning ▁Mong m ins im se o ▁and ▁on ▁administration ▁G ye ongs es ir ye ong ▁( 1 8 2 2 ). ▁ ▁Das an ▁remained ▁in ▁ex ile ▁in ▁Gang jin ▁until ▁ 1 8 1 8 , ▁when ▁he ▁was ▁allowed ▁to ▁return ▁to ▁his ▁family ▁home ▁near ▁Se oul . ▁Att empt s ▁to ▁bring ▁him ▁back ▁into ▁government ▁service ▁were ▁blocked ▁by ▁fa ction al ▁politics . ▁He ▁used ▁Ye oy ud ang ▁as ▁his ▁final ▁pen - name , ▁it ▁was