YIN AND YANG-THE LAW OF NATURE

     Yin and yang, which come from ancient Chinese philosophy, are a general term 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", (Dao 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 dynamic, bright, hot, functional, etc..., pertain to the category of yang, while those that are static, dark, cold, substantial, etc. , pertain to that of yin. The yin-yang theory holds that the development and changes of everything in the universe result from the unity of opposites be tween yin and yang. Su Wen says: "yin and yang are the law of heaven and earth, 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.中国中医科学院西苑医院消化科（脾胃病科）李博

l. The Unity of Opposition Between Yin and Yang

  By the opposition between yin and yang is meant that all things or phenomena in nature have 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（P29） 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 that the dynamic equilibrium can be established. For instance, in the nature 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 autunm 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 not 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 will be broken, thus bringing about dissociation of yin and yang, depletion of essence-qi. and even an end of one"s life.

3. Waning and waxing of yin and yang

  Yin and yang always coexist in a dynamic eqilibrium in which one waxes while the other wanes. In other words, waning of yin will lead to waxing of yang and vice versa. Take the seasonal and climatic variations for example, it gets warm from winter to spring, and hot from spring to sumner. This is the process of "yang waxing and yin waning," Conversely, it gets cool from summer to autumn, and cold from autumn to winter-the process known as "yin waxing and yang waning". Under normnl conditions, the waning-waxing relation of yin and yang is in a state of relative balance.If this relation goes beyond normal limits, the relativc balance of yin and yang will not be maintained, thus resulting in either excess or deficiency of yin or yang and the occurrence of diseasc, so far as to endanger one"s life.

4. Transformation of yin and yang

    In given conditions, either yin or yang may transform into its opposite, i. e. yin may be transformed into yang and yang into yin. If the waning-waxing of yin and yang is said to be a process of quantitative change, then that of their inter-transformation pertains to a qualitative change based on the quantitative chang. Such a process is mostly a gradual one from quantitative to qualitative change. Su Wen states: "Extreme yin gives rise to yang, while extreme yang gives rise to yin." "Extreme cold brings on heat, while extreme heat brings on cold". This is termed"Things will develop in the opposite direction when they become extreme." Pathologically, the yin syndromes can be transformmed into yang syndromes, and vice versa. It must be pointed out that the decisive factor of the mutual transformation is the conditions, including internal and external conditions, without which such transformation will be by no means likely to occur.

   The above statement is the basic content of yin-yang theory, which is also illustrated by "Taijitu" (Yin-Yang Diagram) below. In the diagram the white part indicates yang, and the black part yin. The relationships between the two are both opposite and complementary to each other. The white spot within yin shows the yang within yin, while the black spot does yin within yang. Meanwhile, they contain the potential for Intertransformation and inter-wane-wax between yin and yang.

   From the above, we can see that the content of yin-yang theory is composed of four aspects, among which the opposition and waxing-waning contain the opposite of contradiction; the interdependence and transformation contain the unity of contradiction; waxing-waning and transformation contain quantitative change and qualitative change. The former is the precondition for the latter, the latter is the outcome of the former.