Li Ao is perhaps the thinker in Tang China (618–907) who contributed most to a new version of Confucian philosophy that addressed issues of human nature and spiritual cultivation. By Li's time, questions in this area had been left to Buddhism and Daoism for centuries, whereas the intellectual elite in general considered Confucianism solely the authority in family and political lives. Li's importance as a thinker comes entirely from a single treatise: the Fuxing shu (Writings on returning to one's true nature). It is arguably the first post-Han (206 BCE–220 CE) text that gave an original treatment on the topics of human nature and spirituality from a Confucian stance.

The theme of the Fuxing shu is how to become a sage, the Confucian ideal of personality. Li holds that a sage is a person who has realized his "nature" (xing), the character of which can be...