Cheng Yi was a prominent Chinese philosopher during the Song Dynasty, known for his contributions to Neo-Confucianism. Born in 1033, he emphasized the importance of moral self-cultivation and the study of Confucian texts. Cheng Yi's ideas focused on the relationship between human nature and the universe, advocating for a harmonious existence. He was also a key figure in the development of the Li (principle) and Qi (vital energy) theory, which became central to later Neo-Confucian thought. Cheng Yi's teachings influenced many scholars, including his younger brother, Cheng Hao, and left a lasting impact on Chinese philosophy.