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Yīn and yáng both prevent their complement from becoming disproportionately strong. When either pole weakens, the other may grow stronger. Thus, in the body, when yīn humor is depleted, yáng qì appears relatively stronger, and manifests in the form of heat (vacuity heat). In the body, yīn and yáng counterbalance each other. A deficit of one naturally leads to a surfeit of the other, while a surfeit of one will weaken the other. In both cases, yīn and yáng no longer counterbalance each other, and disease arises as a result. In medicine, the notion of counterbalancing is widely applied in physiology, pathology, and therapy.

In physiology, for example, liver yīn counterbalances liver yáng, preventing it from becoming too strong. If liver yīn becomes insufficient and fails to counterbalance its complement, ascendant hyperactivity of liver yáng develops.

In the relationship of evils and the human body, yáng evils invading the body will cause a surfeit of yáng, which may lead to damage to yīn humor and the emergence of a heat pattern. Conversely, a yīn evil entering the body will lead to a surfeit of yīn, causing damage to the body’s yáng qì and the emergence of a cold pattern. These processes are described in Plain Questions (素问 sù wèn) in the following way: If yáng abounds yīn ails, and if yīn abounds, yáng ails; when yáng prevails there is heat, and when yīn prevails there is cold.

In therapy, if a disease is caused by heat evil, it is treated with cool or cold medicinals according to the principle that cold can counteract heat, meaning yīn medicinals combat yáng evils. Similarly, diseases caused by cold evil are treated with warm or hot medicinals, since heat can overcome cold, i.e., yáng medicinals can combat yīn evils. This is summed up in a guiding principle of therapy, heat is treated with cold; cold is treated with heat. It is most often applied in patterns of repletion characterized by a surfeit of either yīn or yáng. In conditions caused by deficit of yīn or yáng, the opposing complement is no longer kept in check and becomes disproportionately strong. If yīn is vacuous, yáng is no longer kept in check and its strength will grow out of proportion to that of yīn. Such a condition is at root a yīn vacuity, manifesting itself as vacuity heat. For this reason, treatment by draining fire and clearing heat alone is not only ineffective but also detrimental to the patient’s health. It is replaced by a method such as enriching yīn and downbearing fire, or fostering yīn and subduing yáng, whereby clearing heat and draining fire are secondary to enriching yīn. By supplementing yīn, the yáng surfeit will naturally diminish. This explains the principle invigorate the governor of water to restrain the light of yáng. In the reverse situation, where yáng is vacuous and fails to keep yīn in check, there is exuberant internal yīn cold, manifesting in such forms as clear-grain diarrhea, fifth-watch diarrhea, and water swelling. Here, treatment should aim not simply at dispersing cold evil but also at supplying the yáng vacuity through such methods as assisting yáng, boosting fire, and supplementing qì. This demonstrates the principle that where warming is to no avail, fire is lacking and boost the source of fire to disperse the shroud of yīn. It is important to note the difference between the natural ebb and flow of yīn and yáng and a surfeit of one or the other complement.

Waxing and waning of yīn and yáng refers to their normal relationship in the human body, which is one of constant fluctuation, rather than a rigid, immutable balance. When yīn rises, yáng ebbs, and when yáng swells, yīn subsides. This constant fluctuation is apparent in all the body’s functions, such as fluid production and metabolism, the role of the five viscera in storing essential qì, and the role of the six bowels in conveyance and transformation of food. By contrast, deficit and surfeit denote the disturbance of the normal relative balance and failure to rectify the imbalance immediately. This is known as imbalance of yīn and yáng, which is the underlying cause of all disease.

See yīn and yáng.

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