Yīn and yáng are rooted in each other

Yīn and yáng are mutually dependent. Yīn is rooted in yáng and yáng is rooted in yīn. The notion of interdependence means that neither phenomenon of a yīn-yáng pair can exist without the other. Yīn exists by virtue of yáng, and yáng exists by virtue of yīn. Light (yáng) cannot exist without darkness (yīn), and darkness cannot exist without light. Similarly, activity (yáng) cannot exist without rest (yīn). Note that in modern Western thought, darkness and rest are defined by the absence of their opposites; in Chinese thought, both darkness and light, rest and activity are considered equally real.

In medicine, the concept of interdependence of yīn and yáng is widely used in physiology, pathology, and treatment.

Blood and qì, two fundamental elements of the human body, provide an example: blood is yīn and qì is yáng. It is said that qì engenders blood, i.e., blood formation relies on the power of qì to move and transform food; qì moves the blood, meaning that blood circulation relies on the warming and driving power of qì. Furthermore, qì contains the blood, i.e., it keeps the blood within the vessels. The functions of engendering, moving, and containing the blood are summed up in the phrase, qì is the commander of the blood. Conversely, qì is dependent on the provision of adequate nutrition by the blood; thus it is said that qì has its abode in the blood, and blood is the mother of qì. Because qì has the power to engender blood, treatment of blood vacuity involves dual supplementation of qì and blood. Massive bleeding, where qì deserts with the blood, is first treated by boosting qì, since blood-nourishing formulas should not be administered until qì is secured. Similarly, formulas used to treat qì vacuity often include blood-nourishing medicinals to enhance qì supplementation.

Another example of the interdependence of yīn and yáng, seen in the development of diseases, is the principle that detriment to yīn affects yáng and detriment to yáng affects yīn. Since without yáng, yīn cannot be born, when yáng vacuity reaches a certain point, the production of yīn humor is affected, and yīn also becomes vacuous. Most cases of what Western medicine calls chronic nephritis indicate yáng vacuityand are characterized by water swelling due to the inability of the kidney to transform fluids. However, when the yáng vacuity reaches a certain point, fluid formation is affected and a yīn vacuity pattern evolves. This demonstrates the principle that detriment to yáng affects yīn. Similarly, yīn vacuity, when reaching a certain peak, lead to simultaneous yáng vacuity, since without yīn, yáng cannot arise. What is termed hypertension in Western medicine usually corresponds to hyperactivity of yáng caused by vacuity of yīn. In severe cases, this condition may develop into a dual yīn-yáng vacuity, illustrating the principle that detriment to yīn affects yáng.

See yīn and yáng.