The Ming and Qing dynasties were the key periods for constructing the theory of tongue diagnosis in traditional Chinese medicine (TCM). From the relevant literature in this period, we can see the process of integrating tongue diagnosis into TCM theory. Metaphors help with explaining and constructing new theories. Therefore, this paper uses the metaphor cognitive theory to analyze the literature related to tongue diagnosis in the Ming and Qing dynasties, suggesting that the five elements play a major role in explaining the clinical significance of tongue color and describing the color of the normal tongue from red to light red with "red is the fire of the heart" and the location and functional relationship among the viscera. In exogenous diseases, the "water is cold and fire is hot" in the five elements is used to explain the cold and hot pathogenesis of tongue color, and the lungs and stomach correspond to white fur and yellow fur, respectively, to distinguish the exterior and interior locations of disease. Among internal diseases, the disease location of the viscera and the pathogenesis of deficiency and excess reflected by the tongue color are determined by the five colors corresponding to the viscera. From the perspective of "burnt charcoal is black" and color composition, the ancient physicians explained the tongue diagnosis experience of "black tongue is hot" beyond the five color categories. This study reveals the generalization and refinement of the clinical experience of tongue diagnosis by TCM theory and the important role of metaphors in this process, which provides a reference for understanding more clinical experiences from the perspective of TCM.

tongue diagnosis;color;metaphor;traditional Chinese medicine theory;five elements;exogenous diseases;internal diseases