The thesis that ¡°human nature is good¡± was a key doctrinal foundation of the Daoxue tradition since its beginning. Nonetheless, the process of verifying this thesis revealed intricate problems concerning the moral definition of human nature and its categorization. In particular, because of its implications of universality and pure goodness, this thesis seems to contradict the empirical facts of the occurrence of immorality and the individual disparities in moral characters. In order to address such problems, Zhu Xi ñ¹ýø (1120-1200) adopted the ideas of qizhi zhi xing Ñ¨òõñýàõ and the unity of human nature from Zhang Zai íåî° (1020-1077) and the Cheng brothers through re-conceptualizations. First, Zhu Xi re-interpreted qizhi zhi xing in the sense of the mutual non-separability (bu xiang li ÜôßÓ×î) of the universality of human nature and the personal qualities of qizhi. Thus, he provided an explanation for observed disparities in moral character in the light of the influence of the personal quality of the endowed qizhi on the manifestation process of human nature. At the same time, from the perspective of the mutual non-fusionablity (bu xiang za ÜôßÓíÚ), he confined the range of qizhi¡¯s influence to the manifestation process alone, rather than to human nature itself, thus making human nature itself conceptually free from the influence of qizhi and theoretically capable of retaining its quality of pure goodness. Second, to re-establish the thesis that human nature is good on a more solid ground, Zhu needed to redefine the relationship between human nature and sensory desires. Because the latter had been conventionally marked out as the primary cause of immorality, they could not be simply integrated into one concept. On the other hand, when separating them into two distinct entities, this would divide the concept of human nature itself into the two subcategories as the distinct origins of morality and immorality, which also would make the moral definition of human nature ambiguous. To solve this problem, on the side of the all-embracing unity of human nature, Zhu included sensory desires into the category of human nature and explained their tendencies toward immorality as nonessential, accidental anomalies occurring in the process of the manifestation of human natur as the sole ultimate origin of all, rather than directly stemming from human nature itself. By addressing these two remaining problems, Zhu eventually established Mengzi¡¯s thesis of the goodness of human nature as the orthodox view on this subject for the first time in Chinese intellectual history.