阴阳

Yin (陰) and yang (陽) are the most basic categorical pair in classical Chinese thought. The Yellow Emperor's Inner Classic (《黃帝內經》, Huángdì Nèijīng), the foundational text of Chinese medicine, takes the pair as the organizing principle for understanding the human body, explaining physiology and pathology, and guiding diagnosis and treatment. "Yin and yang are the way of heaven and earth, the great net of all things, the parents of change, the root and beginning of life and death" — so the Inner Classic declares their universal scope.

Yin and yang are not two substances but a pair of relational attributes. What is bright, outward, ascending, warm, and active is classified as yang; what is shaded, inward, descending, cool, and quiet is classified as yin. Crucially, the pair is recursive: any single thing can itself be subdivided. Daytime is yang relative to night, but the morning is the yang within the yang of the day, while the afternoon is the yin within the yang.

The relationship is dynamic, not a static binary. Yin and yang depend on each other (互根, mutual rooting — neither can exist without the other), restrain each other, wax and wane in turn, and under certain conditions transform into one another. In Chinese medicine, health is defined as the relative balance of yin and yang within the organism. Disease, at its most general level, is always an imbalance of yin and yang; the treatment principles of warming, cooling, supplementing, and draining can all be traced back to this single root concept.

In the body, the back is yang and the abdomen is yin; the six fu (yang organs, hollow conduits of digestion and excretion) are yang and the five zang (yin organs, the storing organs of the interior) are yin; qi is yang and blood is yin; function is yang and substance is yin. Within each organ the analysis can be repeated: the Heart has its own Heart-yin and Heart-yang, the Kidney its Kidney-yin and Kidney-yang.

Physiologically, yang warms and propels, while yin moistens and consolidates. Yang qi is like the sun, warming and circulating throughout the body; yin blood and yin fluids are like water, nourishing and giving form. The two are complementary: "Yin within is the guardian of yang; yang without is the agent of yin."

Pathologically, excess and deficiency of yin or yang each produce their characteristic patterns. Excess yang produces heat (excess heat); excess yin produces cold (excess cold). Deficient yang also produces cold — but it is empty cold, with cold signs but no concrete pathogen — while deficient yin produces heat, classically the afternoon tidal fevers, night sweats, and "five-center heat" (warmth in the palms, soles, and chest) familiar to every clinician. Identifying which side of the pair is in excess or deficiency is the first step in formulating any treatment.