This article aims to investigate the issues of the ethical function of emotion in Dasan’s annotations of Confucian classics. Defining nature as “preference,” Dasan argued that the specific content of nature is to “like good and dislike bad.” Verifying the existence of such nature based on his own psychological experiences and the existing canon of literature, he attempted to prove the presence and universality of moral emotions, especially in ordinary dialogue, relying on psychological responses to specific events and the notion of human nature. Since identical emotions can lead to contrasting actions, depending on whether they “achieve harmony in moderation,” emotions should be properly adjusted. To achieve this, Dasan stressed the importance of sincerity and proposed that people, as sincere actors, exert every effort for religious cultivation by serving Sangje (Lord on High). Dasan’s theory on emotions offers a basis for empirically resolving the fundamental problems of Confucian ethics, and his notion of emotions has significance in shifting philosophical concerns from the metaphysical sphere to the everyday lived world.