Yin and yang

The Suwen, or Basic Questions, is the first half of the Huangdi Neijing, the Yellow Emperor’s Inner Classic, which Britannica’s history of medicine dates in its present form to no earlier than the 3rd century BCE. Its fifth chapter opens on yin and yang. In Richard Bertschinger’s translation, the pair “form the way of the skies and earth”, are “the father and mother of all change and transformation”, and are the root a physician must seek: “In treating sickness you must seek for the root.” The commentator Zhang Zhicong, whose note Bertschinger prints beside the line, reads the root as yin and yang themselves, since the organs, qi and blood, inner and outer, upper and lower all find their root in the pair. This page follows yin and yang from what the words first meant, through the four relationships the theory gives them, to the three places they shape acupuncture: the organs, the channels and the first sorting of a patient’s findings. Throughout, the texts are the subject. None of it is a finding of physiology.

What the words meant first

The characters for yin and yang appear on oracle bones, the animal bones used in Chinese divination from at least the 14th century BCE. Robin Wang’s entry on yinyang in the Internet Encyclopedia of Philosophy notes that in those inscriptions they describe the weather and the movement of the sun: daylight is yang and its absence at night is yin. The Shuowen jiezi, a dictionary compiled around 100 CE, glosses yin as a closed door, darkness, the south bank of a river and the north side of a mountain, and yang as height, brightness and the south side of a mountain. Wang finds no ranking of one above the other in any of the uses she surveys.

The pair later gave its name to a school. The historian Sima Tan, writing around 110 BCE, listed a yinyang school alongside five others, among them the Confucians and the Daoists, and the Shiji, the Han dynasty’s Records of the Historian, names Zou Yan as its representative. None of his writing survives. Britannica’s entry on Zou Yan, a cosmologist of the state of Qi in the 3rd century BCE, attributes to him the joining of yinyang to the theory of the five phases, which The five phases covers.

Four relationships

The WHO’s International Standard Terminologies on Traditional Medicine in the Western Pacific Region, published in 2007 to fix English terms for the field, defines yin and yang as “the general descriptive terms for the two opposite, complementary and inter-related cosmic forces found in all matter in nature”. It names four relationships between them. The table gives each with the WHO’s definition and a line from Suwen chapter 5, in Bertschinger’s translation, that states it.

The classification repeats inside itself. The WHO’s example is the day: daytime is yang relative to night, but within it the period from dawn to noon is yang within yang and the period from noon to nightfall is yin within yang, while the hours from midnight to dawn are yang within yin. Anything already classed as yin or yang can be divided again in the same way.

Transformation has a pathological form. The WHO terminology describes “extreme yin resembling yang”, in which exuberant yin in the interior forces yang qi out to the surface and produces a pattern of true cold with false heat, and the reverse, “extreme yang resembling yin”, a pattern of true heat with false cold. The commentator Li Zhongzi, in Bertschinger’s selection, warns that a physician who misses the interchange and prescribes regardless acts “like deepening the water or building up the fire”.

In the body

Suwen chapter 5 maps the pair onto the body by direction and density. “The clear Yang comes out from the upper openings, the turbid Yin comes out from the lower openings”; the clear yang “develops into the texture of the skin” and “gives substance to the four limbs”, while the turbid yin “runs back to the five zang” and “returns to the six fu”. Wang’s IEP entry summarises the Neijing’s classification of the organs: the five zang, which store vital substance and qi, are yin; the six fu, which transport and digest food, are yang; and each organ divides again, its activity yang and its substance yin.

The WHO terminology carries that division into every organ. Kidney yin has “a moistening, nourishing and cooling effect on all organs”; kidney yang “warms and activates all organs”. Within the zang it also classes the heart and liver as yang and the spleen, lung and kidney as yin. The zang-fu organs describes the organs themselves. Deficiency of either side is a pattern in its own right: the WHO defines yin deficiency as a “diminished moistening, calming, downbearing and yang-inhibiting function, leading to relative hyperactivity of yang qi”. AcuiQ’s index carries yin deficiency, yang deficiency and kidney yin deficiency as search terms because published prescriptions name them, and Patterns, symptoms and what you can look up explains why pattern names sit in a symptom index.

In the channels

Each of the twelve regular channels carries one of six names, three yin and three yang, which the WHO terminology defines: greater yin (taiyin), lesser yin (shaoyin) and reverting yin (jueyin); greater yang (taiyang), yang brightness (yangming) and lesser yang (shaoyang). Each name covers one channel of the hand and one of the foot, so that greater yin, for example, names both the lung and the spleen channels. The channel system, counted gives every channel’s classical name.

The assignment also fixes where each channel runs and in which direction. In the WHO definitions, the three yin channels of the hand run along the front of the arm from the chest to the hand, and the three yang channels of the hand along the back of the arm from the hand to the head. The three yang channels of the foot run from the head down to the feet, and the three yin channels of the foot run up the inner side of the leg from the feet to the abdomen and chest.

In diagnosis: the eight principles

The WHO terminology lists eight guiding principles of pattern identification: yin and yang, exterior and interior, cold and heat, deficiency and excess. Yin and yang group the other six. Interior, cold and deficiency patterns are yin, with “inhibitory, hypofunctional, quiescent or dimmed manifestations”; exterior, heat and excess patterns are yang, with “excitatory, hyperfunctional, restless or bright manifestations”. The findings these principles sort come from the four examinations: looking, listening and smelling, asking, and palpating.

What research has tested

Ted Kaptchuk’s 2002 review in Annals of Internal Medicine describes yin-yang, qi, dampness and wind as East Asian conceptual frameworks that emphasise the reliability of ordinary human sensory awareness, and notes that many physicians who practise acupuncture reject such prescientific notions. What has been measured is whether practitioners who apply the framework reach the same conclusion about the same patient. A 2019 systematic review by Jacobson and colleagues found 21 studies of agreement between Chinese medicine practitioners who examined people in person. Across the 9 that reported simple agreement it averaged 57%, and across the 7 that reported Cohen’s kappa, a measure that discounts agreement expected by chance, it averaged 0.34. The authors judged agreement low to moderate with a few exceptions, and found significant deficits of method and reporting in most of the studies.

What the sources cannot tell you

Bertschinger describes the Suwen as assembled by Han scholars who used yinyang and wuxing thinking to categorise and synthesise all their data, and the commentaries printed beside it are later still: he reads Li Zhongzi and Zhang Jiebin as reflecting the warming and tonifying school of the Ming herbalists. A line of chapter 5 read through them shows how the tradition came to read it, not what its first authors meant. AcuiQ records which points a study prescribed and what the authors said they were treating. A prescription filed under yin deficiency means its authors called the condition that; AcuiQ does not check the differentiation behind it, as What AcuiQ does not do sets out.