In Korean Medicine (KM), the Yin-Yang doctrine is still used as a theoretical tool for understanding and explaining the clinical experiences. However, as the traditional culture declined in East Asia and the scientific culture took over, there was an increased negative view on the Yin-Yang doctrine, and thus a heightened distrust over KM. For KM to survive in an unfamiliar culture of science, a novel outlook on the Yin-Yang doctrine is needed. In this sense, I consider a thoroughly medical take on the Yin-Yang doctrine to be most important. The focus needs to be on the goals of medicine: this includes riddance of any discourses on Yin-Yang that cannot contribute to the goals, and an enhancement of the Yin-Yang concept as a rational and scientific terminology. One way to achieve this is by understanding Yin-Yang as a type of metaphor. The Yin-Yang doctrine that is utilized in KM corresponds well to the conceptual metaphor suggested by Lakoff and Johnson. As a metaphor, the Yin-Yang concept plays a role in structuring the target domain, that is life phenomena, metaphorically. Through the Yin-Yang metaphors, the life phenomena are understood as the Yin-Yang phenomena, and are systematically organized by the subcategories contained in the Yin-Yang doctrine. Understanding Yin-Yang as a metaphor is a good way to enhance the Yin-Yang concept and doctrine as a rational terminology and method.