1. Inter-consuming supporting relationship of yin and yang (waxing-warning of yin and yang)
The balance between yin and yang is not static, but dynamic. Anabolism and catabolism do not always proceed at the same speed or to the same degree. During daytime activities there is more catabolism than anabolism, while during rest or sleep at night there is more anabolism than catabolism. That is to say, there is an increase of materials at night along with a decrease of activities, and there is an increase of activities in the daytime accompanied by a decrease of its material basis. Of the two opposites of a single entity, increase or excess of the one is usually associated with decrease or deficiency of the other. As stated in The Medical Classic of the Yellow Emperor, "Consumption of yin leads to gaining of yang" and "consumption of yang leads to gaining of yin," and the whole process is called "waxing-waning of yin and yang" for short. In normal conditions, the waxing-waning of yin and yang is in a state of relative balance. If it goes beyond physiological limits, the yin-yang balance will be impaired, resulting in disease.
2. Opposition of yin and yang
The theory of yin-yang holds that everything in the world has two opposite aspects, namely, yin and yang. The yin or yang aspect within any phenomenon will restrict the other through opposition. Take the human metabolism as an example. Catabolism (destructive metabolism) and anabolism (constructive metabolism) are the two aspects of metabolism. The former may pertain to yang, while the latter to yin. They are apparently in opposition, but they should be kept in a dynamic balance so as to guarantee the normal metabolism.
3. Interdependence of yin and yang
Yin and yang oppose each other, but at the same time they are mutually dependent. Without anabolism there would be no catabolism, and without catabolism there would be no anabolism. In other words, if no living tissue could be formed through anabolism, the human body would cease to exist, and if no energy could be changed from the living tissue, the human body would die. Therefore, in the theory of yin-yang, it is said that yang exists with yin as its prerequisite, and yin exists with yang as its prerequisite.
4. Infinite divisibility of yin and yang
The divisibility of yin and yang is infinite. As stated in The Medical Classic of the Yellow Emperor, "Yin and yang can be counted in tens, and can be extended to hundreds, thousands or infinity." For example, day and night can be divided into yin and yang, in which day is yang, while night is yin. Each can be further divided into yin and yang: morning is yang in yang, while afternoon is yin in yang; the period from nightfall to midnight is yin in yin, while the period of the small hours is yang in yin. If we take metabolism as an example, the divisibility of yin and yang is more explicit. Either catabolism or anabolism can be divided into opposite processes, and each process can be further divided into opposite aspects. Nutrients (yin) and energy (yang) are a pair of opposites. A part of energy released in catabolism turns into heat for maintaining the body temperature (yang in yang), and the rest is stored in adenosine triphosphate (ATP) (yin in yang). The energy released from ATP is further divided into two parts: one is used for muscular movement as mechanical energy (pertaining to yang), and the other is stored in creatine phosphate (pertaining to yin).
5. Inter-transformation of yin and yang
During metabolism there is inter-transformation of the living tissue for construction and energy for functional activities. Normally, the inter-transformation takes place within a limited range, forming a dynamic balance. However, in certain circumstances, either yin or yang may be transformed into its opposite. In an acute febrile disease, there is abnormally increased catabolism to produce a high fever. After persistent high fever, exhaustion may occur, with a sudden drop in body temperature, pallor, cold limbs and profuse sweating. This is a common example to illustrate transformation of yang (excess of energy with a heat syndrome) into yin (exhaustion of energy with a cold syndrome).