Yi Saek (, 17 June 1328 – 17 June 1396), also known by his art name Mokŭn, was a Korean writer and poet. His family belonged to the Hansan Yi clan. 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 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. 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.

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.

Many of his disciples, such as Chŏng To-jŏn and Kwŏn Kŭn, 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 Chŏng To-jŏn 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 speculate that he was murdered while crossing a bridge, like Chŏng Mong-ju (four years earlier in 1392). When he was offered the position of Prime Minister by Yi Sŏng-gye, Yi Saek turned the offer down and told him that he could not serve two kings (of Goryeo and Joseon). Yi Sŏng-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.

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

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李穡（；1328年 - 1396年），字穎叔，號牧隱，本貫韓山李氏，高麗楊廣道韓州人，高麗王朝末期的學者兼政治家、儒學家。謚號文靖。

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