Classification into Six Levels (Lü-jing)

diagnostic systems

The six-level system described in the *Shanghan Lun* explains how a “cold” disease develops as it penetrates deeper through six stages: Taiyang, Yangming, Shao-yang (the three Yang levels), and Tai-yin, Shao-yin, and Jue-yin (the three Yin levels). The yang levels correspond to the more superficial and “full” stages, while the yin levels correspond to the deeper and “empty” stages. Tai-yang is the external stage characterized by chills and body aches; yang-ming is the stage of internal heat; and shao-yang is the “semi-external” stage. Each stage has its own specific formulas; this system remains the foundation for treating acute external diseases.