Late-Goryeo civil official and Neo-Confucian scholar; one of the Three Recluses (三隱) (pen name Mogeun)

| Life | 1328 ~ 1396 |
| Era | 고려 말 |
| Clan | 한산 이씨 |
Yi Saek was a civil official and Neo-Confucian scholar of late Goryeo; his pen name was Mogeun (牧隱). Following his father Yi Gok (李穀), he passed the civil service examination in Yuan China, and after returning home served as headmaster of the Confucian academy (Seonggyun-gwan daesaseong), contributing greatly to spreading and expanding Neo-Confucianism in Goryeo.
He nurtured Jeong Mong-ju, Jeong Do-jeon, Gwon Geun, Yi Sung-in, and others as his disciples, becoming the spiritual leader of the emerging literati of late Goryeo. He served as jeongdang munhak and panmunhabusa and was enfeoffed as Lord of Hansan. Together with Jeong Mong-ju and Gil Jae, he is counted among the "Three Recluses" (三隱) who kept their fidelity to Goryeo.
He did not cooperate in the founding of Joseon, and after suffering exile and hardship in his later years, he died at Yeogang (Yeoju) in 1396. He left the vast poetry and prose collections Mogeun sigo (牧隱詩藁) and Mogeun mun'go (牧隱文藁); his posthumous title was Munjeong (文靖).
Yeongmo-ri, Gisan-myeon, Seocheon County, South Chungcheong Province (near Munheon Academy)
He had sons Yi Jong-deok, Yi Jong-hak, and Yi Jong-seon, and his grandson Yi Gae (李塏) was one of the Six Martyred Ministers who died plotting to restore King Danjong.