八纲辨证

The Eight Principles organize every clinical presentation into four pairs of opposing categories: yin/yang, exterior/interior, cold/heat, and vacuity/excess. Yin and yang are the overarching pair — exterior, heat, and excess are forms of yang; interior, cold, and vacuity are forms of yin. The other three pairs serve to specify the location of disease (exterior or interior), its essential nature (cold or heat), and the relative strength of upright qi against the pathogen (vacuity or excess). Any concrete pattern, no matter how complex, can be sketched at first pass in these eight characters.

While the name "Eight Principles" was not formally consolidated until the Ming and Qing dynasties, the content is woven throughout the Yellow Emperor's Inner Classic and Zhang Zhongjing's Treatise on Cold Damage. Modern textbooks of Chinese medical diagnosis place the Eight Principles in the first chapter precisely because they are the simplest and most universal entry point into the discipline.

Exterior and interior (表/里) locate the disease. Exterior patterns arise when an external pathogen attacks the surface of the body and produce aversion to cold with fever, headache and body aches, and a floating pulse; the illness is recent and shallow. Interior patterns are situated in the zang-fu organs, qi, and blood; they are deeper, generally graver, and slower to resolve.

Cold and heat (寒/热) name the essential nature of the disturbance. A cold pattern is marked by aversion to cold and preference for warmth, a pale complexion, absence of thirst, copious clear urine, a pale tongue with a white coating, and a slow pulse. A heat pattern is marked by fever and aversion to heat, a flushed face and red eyes, thirst with a preference for cold drinks, constipation and reddish urine, a red tongue with a yellow coating, and a rapid pulse.

Vacuity and excess (虚/实) name the relative strength of the upright qi versus the pathogen. Vacuity patterns reflect insufficiency of the body's own substances and functions: weariness, breathlessness, spontaneous sweating, a pale tongue, and a weak pulse. Excess patterns reflect a vigorous pathogen meeting an as-yet-undepleted body: distending pain that resists pressure, constipation and dark urine, a thick tongue coating, and a forceful pulse.

Yin and yang are the overarching pair: interior, cold, and vacuity are all forms of yin, while exterior, heat, and excess are all forms of yang. In practice the clinician first sorts the case into yin or yang, then identifies its location, nature, and strength, and only then refines the diagnosis into the more detailed languages of organ, qi-blood, or channel pattern differentiation.