This Chapter, from the Yellow Emperor's Classic of Internal Medicine, details a system of classifying individuals into twenty-five types based on the interplay of five elemental phases (wood, fire, earth, metal, water) and yin-yang principles. Each type is characterized by specific physical features, temperaments, and seasonal vulnerabilities. The text explains how to identify these types through observation and how this knowledge informs treatment by balancing blood and qi. It emphasizes the secrecy and importance of this ancient medical knowledge, passed down through generations of masters. Finally, the text connects physical characteristics to the flow of qi and blood, emphasizing the importance of understanding these connections for accurate diagnosis and treatment.