Yi Saek (Korean: 이색, 17 June 1328 – 17 June 1396^[a]), also known by his art name Mogeun (목은), was a Korean writer and poet. His family belonged to the Hansan Yi clan.^[1] Yi Saek played a crucial role in the introduction and localisation of philosophy of Zhu Xi. He studied Neo-Confucianism in Yuan Dynasty China and opened an academy after his return to Goryeo, and from his academy the founders of Joseon Dynasty were educated.

Yi Saek
Korean name
Hangul	이색
Hanja	李穡
Revised Romanization	I Saek
McCune–Reischauer	I Saek
Art name
Hangul	목은
Hanja	牧隱
Revised Romanization	Mogeun
McCune–Reischauer	Mokŭn
Courtesy name
Hangul	영숙
Hanja	穎叔
Revised Romanization	Yeongsuk
McCune–Reischauer	Yŏngsuk

Yi was one of the most significant cultural figures in the country. Aiming to solve the social problems facing Goryeo, Yi fulfilled his responsibilities as a scholar, educator and politician based on his experience studying at Guozijian, a national Chinese university.^[2] While leading Sungkyunkwan academy, Yi developed a curriculum that reformed the academy's evaluation methods, which was innovative even by today's standards. Yi Saek played a very important role in introducing Neo-Confucianism to Goryeo, which was the theoretical background for the founding of Joseon.^[2]

Yi Saek was patronized by kings during the Koryo period (918–1392). He promoted education based on Confucian texts. Yi Saek is also responsible for establishing the Confucian tradition of public mourning. He favored Confucianism in public affairs. Towards the end of his life, Yi Saek was the respected head of the Confucian National Academy. He is remembered as one of the "Three Hermit Scholars" devoted to Confucian principles.^[3]

Many of his disciples, such as Jeong Do-jeon and Gwon Geun, used Neo-Confucianism as the ideological basis for overthrowing the Buddhist kingdom of Goryeo and establishing Confucian Joseon. However, Yi Saek himself remained loyal to the Goryeo Dynasty and didn't believe the wiping out of Buddhism, as Jeong Do-jeon insisted, would be of any benefit. Yi Saek believed in the co-existence of the "Three Disciplines": Confucianism, Buddhism and Taoism. Yi Saek resigned from all political positions after the founding of the Joseon Dynasty.

Some^[who?] speculate that he was murdered while crossing a bridge, like Jeong Mong-ju (four years earlier in 1392). When he was offered the position of Prime Minister by Yi Seong-gye, Yi Saek turned the offer down and told him that he could not serve two kings (of Goryeo and Joseon). Yi Seong-gye subsequently ordered his men to kill him if he was not able to cross the bridge in time, and if he does cross it in time, to let him go. He died because he didn't cross the bridge in time.^{[citation needed]}

Yi Saek left various poetry, essays and letters compiled in The Collected Works of Mogeun.