Chinese philosopher who influenced the development of the rationalist school of Neo-Confucianism. His statement “Principle is one but its manifestations are many” stressed the importance of investigation and contrasted with the introspective idealist Neo-Confucian philosophy of his brother, Cheng Hao.

After Cheng passed his civil service examinations, he served briefly as imperial tutor (1069–70), but his stern conception of morality soon alienated many of those around him, and he resigned. For most of his life he declined high office. Nonetheless, he continued to criticize those in power. As a result, in 1097 his land was confiscated and his teachings barred, and he was banished to Fuzhou, in southeastern China. He was pardoned three years later but was again censured in 1103. He was pardoned a second time, in 1106, shortly before his death. Because people feared to be associated with Cheng, only four individuals attended his funeral.

Both Cheng Hao and Cheng Yi based their philosophies on an understanding of li, a basic force that governs proper behaviour in all things. Cheng Yi—whose philosophy was originally called Daoxue (“School of True Way”) but came to be called Lixue (“School of Universal Principles”)—emphasized that the way to discover li is to investigate the myriad things of the universe in which li is present. He espoused many methods of investigation—induction, deduction, the study of history and other disciplines, and participation in human affairs. Cheng's writings have been gathered in the Yichuan wenji (“Collection of Literary Works by Cheng Yi”), the Jing-shuo (“Explanation of the Classics”), and the Yi zhuan (“Commentary on the Book of Changes”). A decade after Cheng's death, Zhu Xi (1130–1200) began to expand Cheng's ideas into what came to be called the Cheng-Zhu (for its two most important exponents) rationalist school of Chinese philosophy; it dominated official circles until the Chinese Revolution of 1911–12.