Yin and yang, which come from ancient Chinese philosophy, are a general tern for two opposites of interrelated things or phenomena in the natural world. At first, their connotations were quite simple, referring to the two opposite sides of an object. The side facing the sun is yang and the reverse side is yin. In the course of long practice and observation, the ancient Chinese people came to understand that the opposition and wax- wane of yin and yang are inherent in all things. Yi Zhuan says, "Yin and yang are what is called Dao" (Deo means the basic law of the unity of opposites in the universe). In other words, "Everything in the universe contains yin and yang." And they further believed that yin and yang can not only represent two opposite objects but also be used to analyze two opposite aspects existing in a single entity. Generally speaking, things or phenomena which are dynạmic, bright, hot, functional, etc., pertain to the category of yang, while those that are static, dark, cold, substantial, etc., pertain to thạt of yin.
The yin-yang theory holds that the development and changes of everything in the universe result from the unity of opposites between yin and yang. Su Wen says: "Yin and yang are the law of heaven and carth, the principles of all things, the parents of all changes, the origin of life and death..."
The yin-yang theory is an important constituent of the theoretical system of TCM and runs through every aspect of the system. It is used to explain physiology and pathology of the body and to guide clinical diagnosis and treatment. The basic content of yin-yang theory can be summarized as follows.
1. The Unity of Opposition Between Yin and, Yang
By the opposition between yin and yang is meant that all things or phenomena in nature bave two opposite aspects-yin and yang, such as heaven and earth, motion and quiescence, ascending and descending, exiting and entering, day and night, heat and coldness and so on. The former being yang and the latter yin in every pair above. The unity is the outcome of mutual opposition and restriction between yin and yang. Without opposition, there would be no unity; Without mutual opposition, there would be no mutual complement. It is only through this kind of opposition and restriction thạt the dynanic equilibrium can be established. For instance, in the natural world, the motions of celestial bodies, the variations of the four seasons, the alternations of days and nights, as well as sprouting in spring, growing in summer, reaping in autumn and storing in winter, are all the concrete manifestations of the unity of opposites between yin and yang.
TCM believes that the normal physiological functions of the human body result from the opposite and unified relationship between yin and yang. Both of them are always in a state of dynamic balance. Even under normal physiological conditions of the human body, yin and yang can tot be in a state of absolute balance, but in a state of relative balance. If, for any reason, the relative balance is destroyed, there is bound to be excess or deficiency of yin or yang, and then a disease will arise. As is stated in Su Wen, "Yin in excess causing yang disease, while yang in excess leading to yin disease." It is precisely due to the unity of opposites between yin and yang that all things can develop and change ceaselessly and the natural world is perpetually full of life.
2. Interdependence between yin and yang
Yin and yang are opposed to and yet, at the same time, depend on each other. Neither can exist in isolation without its opponent's existence. In other words, without yin there would be no yang, and it's the same the other way round. So either yin or yang is the prerequisite for the other's existence. And this kind of coexistent relationship is stated in TCM, "solitary yin or yang failing to live." This interdependence is also reflected in the relationship between substances and functions. The substance corresponds to yin and the function, to yang. The function is the result of material motion, and nothing in the world is not in a state of motion. Thereby, there is not any substance which can't produce its function and there is also not any function which doesn't originate from the motion of its substance. Therefore, Neijing says: "Yin in the interior is the basis for yang; while yang in the exterior is the activity for yin." "Yin" refers to the material basis of functional activity of yang and "yang" refers to functional activity. The substance and function are interdependent and inseparable." Here is just an imaginable and vivid figure of speech. However, when the interdependent relationships between substances, between functions as well as between substances and functions are abnormal, life activities wil be broken, thus bringing about dissociation of yin and yang, depletion of essence-qi, and even an end of one's life.
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