Cheng Yi (simplified Chinese: 程颐; traditional Chinese: 程頤; pinyin: Chéng Yí; Wade–Giles: Ch'eng I, 1033–1107), courtesy name Zhengshu (正叔), also known as Yichuan Xiansheng (伊川先生), was a Chinese philosopher, politician, essayist, and writer of the Song Dynasty.[1] He worked with his older brother Cheng Hao. Like his brother, he was a student of Zhou Dunyi, a friend of Shao Yong, and a nephew of Zhang Zai. The five of them along with Sima Guang are called the Six Great Masters by his follower Zhu Xi. He became a prominent figure in neo-Confucianism, and the philosophy of Cheng Yi, Cheng Hao and Zhu Xi is referred to as the Cheng–Zhu school or the Rationalistic School.