Toegye was greatest scholar in Korea over long period. There have been no scholars can challenge his academic level. He was very severe in doing scholarly research. Because systematic learning was planed schedule to seek after principle for human. Therefore he did not pardon slapdash and illegality. Although to previous scholars, if they have made mistakes or insufficiency, he must pointed out them. Until Toegye' life, systematic learning have yet not settle down in Choseon dynasty. Then he have try to settle down high standard learning that can be model in after generation, so he have used more strict yardstick. He have estimated previous scholars setting the highest standard with Confucian moral philosophy. Because he have relentlessly disparaged previous scholars who have been imbued with heresy as Buddhism or thoughts of Lao and Zuang. He he have disparaged Cui Zhiyuan, Li Se, for they have flattered Buddha. He have conceded An Xiang's contribution to set up the academy in Korean dynasty and educated many persons with ability, but he have not agreed An Xiang have succeeded Confucion orthodoxy, in opposition to the opinion of Zhou Sepeng. Admitting that Toegye recognized the learning of Zheng Mengzhou, Jin Hongbi, but he intensely expressed his condolences that their works have not been preserved. Toegye have indicated the problems of previous scholars like this. Quan Jin had farfetched problems in research process. Jin Zongzhi attached weight to literature not learning. Zhao Guangzu although have had great contributions which made learning prosper in posterity, but he was annihilated by massacre of scholar, because he was desirous of achievement. Xu Jingde had problems to pride his learning exceedingly for no his deep grasp and to regard Li as Qi. Toegae have esteemed Le Yandi as true scholar who grasped Confucian learning and wrote works to reserved after generation. Toegae have elucidated that he received influence of Le Yandi by putting two charecters 'Hou Xue' on his name. For the first time, Le Yandi could gratify Toegae eyes, as yardsick of Toegae was very severe.