Chengguan (738–839) is the reputed fourth patriarch of the Huayan school of Chinese Buddhism. Revered for his erudite and prolific scholarship, he was among the most influential monks of his time. Although not a direct student of the third Huayan patriarch Fazang (643–712), Chengguan was recognized as Fazang's spiritual successor on the basis of his exceptional learning and prominence, which made him the Huayan tradition's leading figure among his contemporaries. During his formative years, Chengguan became proficient in the scriptures of Buddhism and the doctrines of other Chinese schools, including the Chan school (especially the Niutou and Northern schools), the Tiantai school, and Sanlun. His writings also reveal mastery of the Confucian classics and the works of early Daoist philosophers, especially Laozi and Zhuangzi.
During his long and highly successful monastic career, Chengguan was associated with seven Tang Chinese emperors, and his supporters and admirers included numerous influential officials and literati. The imperial court recognized his achievements by granting him the honorific titles of national teacher and grand recorder of the clergy. Chengguan's magnum opus is the massive Huayan jing suishu yanyi chao (in ninety fascicles), which contains his commentary and subcommentary to the eighty-fascicle translation of the Huayan jing (Flower Garland Scripture). He also wrote other exegetical works, including a commentary on Prajña's forty-fascicle translation of the Huayan jing, and a few shorter tracts. Chengguan's key contribution to the development of Huayan doctrine is the theory of four realms of reality (dharmadhĀtu)—the realms of: (1) individual phenomena (shi fajie); (2) principle (li fajie); (3) nonobstruction between principle and phenomena (lishi wuai fajie); and (4) nonobstruction among phenomena (shishi wuai fajie).
Gregory, Peter N. "Ch'eng-kuan and Hua-yen." In Tsung-mi and the Sinification of Buddhism. Princeton, NJ: Princeton University Press, 1991.