The scholarly consensus on Zhu Xi is that he is brilliant but often unclear, and perhaps even inconsistent, especially with regard to the central concept of li (principle). Any interpretation of his system requires a certain amount of creative reconstruction. The purpose of this essay is to examine the attempts to elucidate Zhu Xi’s doctrine of li over the course of the Four-Seven Debate. It will be seen that the Korean philosophers T’oegye and Yulgok provide competing but complementary explanations of a pivotal issue in Zhu’s moral metaphysics, namely how the li can serve as a standard for moral goodness. However, they both stand in a certain amount of tension, not only with each other but with Zhu himself. It is hoped to show that by juxtaposing the two reconstructions the philosophical problem that arises for Zhu, and for anyone attempting a similar project, can be clarified.