This article analyzes Luo Zheng’an’s theory of xin (mind) and xing (human nature), which is found to be coherent with Zhu Xi’s thought. It argues that Luo’s theory of li-qi (Principle and Matter) which is defined through his theory of mind and human nature, is a modification of Zhu Xi’s doctrine but is not materialism. Luo’s li-qi theory has its characteristics, and his ideas influenced the thought of Yi Yulgok, an important Confucian in the Chosan Dynasty of Korea. Some innovative ideas in Yulgok’s philosophy, such as the idea that qi could not perish, or that the irrational is rational, seem to have been inspired by Luo.