Many of his disciples, such as Jeong Dojeon and Gwon Geun, used Neo-Confucian as the ideological basis for overthrowing Buddhist kingdom of Goryeo and establishing Confucian Joseon. However, Yi Saek himself remained royal to Goryeo dynasty and didn't believe wiping out of Buddhism, as Jeong Dojeon 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 Joseon Dynasty.
Not much is known about how he died, but some say that he was murdered while crossing a bridge. When he was offered the position of prime minister by Yi sung gye, Saek turned the offer down and told him that he could not serve two kings. Yi subsequently ordered his men to kill him if he was not able cross the bridge in time, and if he does cross it in time, to let him go.
Yi Saek left various poetry, essays and letters compiled in The Collected Works of Mokeun.