The yin-yang theory permeates all aspects of the theoretical system of TCM. The theory is used to explain the organic structure, physiological function and pathological changes of the human body. It also serves as a principle to guide clinical diagnosis

l . Explaining the Tissues and Structure of the Human Body

The human body is an integrated whole. All its tissues and structures are organically connected and may be classified as two opposite aspects-yin and yang. That is why Su wen states,”Man, having a form, can not deviate from yin and yang.” In terms of the anatomical locations, the upper part of the body is yang and the lower part is yin; the exterior is yang and the interior, yin; the back is yang and the abdomen, yin; the lateral aspects of the extremities are yang and the medical aspects, yin. Concerning the zang-fu organs, the zang-organs store but not discharge essence-qi and, therefore, they are yin; while the fu-organs transmit and transform food into essence-qi but not store it, and, for this reason, they are yang. Furthermore, each of the zang- or fu-organs can be redivided into yin and yang. For example, heart-yin and heart-yang, kidney-yin and kidney-yang, stomach-yin and stomach-yang, etc. As concerns yin and yang of the meridian-collateral system, there are two categories: yin meridians and yang meridians; yin collaterals and yang collaterals. All of them are opposite pairs. Thus, in line with the yin-yang theory, the unity of opposites between yin and yang exist in the upper and lower, internal and external, front and back parts of the human body, and within all the internal organs as well.

2. Explaining the Physiological Functions of the Human Body

The yin-yang theory believes that the normal life activities of the human body result from the harmonious relation of the unity of opposites between yin and yang. Take the relationship between function and matter for example, function pertains to yang while matter, to yin. Physiological activities of the body are based on matter. Without matter, there would be no sustentation for function activities. And functional activities are the motive power for producing matter. In other words, without functional activities, the metabolism of matter would not be performed. In this way, yin and yang within the human body depend on each other for existence. If yin and yang can’t complement each other and become separated from each other, life will come to an end. So Su Wen says:”The equilibrium of yin and yang makes the vitality well-conserved; the divorce of yin and yang essence-qi exhausted.”

3. Explaining the Pathological Changes

TCM considers that the imbalance between yin and yang is one of the basic pathogenesis of a disease. The occurrence and development of a disease are related to both the vital-qi and pathogenic factors. Although the pathologlical changes which occur in diseases are complicated and changeable, they can still be summarized as excess or deficiency of yin or yang. To be more concrete, “yang excess leads to heat syndrome while yin excess causes cold syndrome”; “yang deficiency results in cold syndrome while yin deficiency causes heat syndromes”; “yang deficiency affects yin while yin deficiency affects yang.”

4. Serving as the Guide to Diagnosis and Treatment 4.指导诊断与治疗

As the imbalance between yin and yang is the root cause for the occurrence and development of a disease, all clinical manifestations, no matter how complicated and changeable they are, can be explained with the aid of yin-yang theory. So TCM holds, “A skilled diagnostician, first of all, differentiates between yin and yang when observing the complexion and feeling the pulse.” In differentiating syndromes, although there are the eight principal syndromes, namely, yin, yang, interior, exterior, cold, heat, deficiency and excess. Yin and yang are regarded as the general principles among the eight ones. According to the yin-yang theory, exterior, heat and excess pertain to yang; while interior, cold and deficiency pertain to yin. When treating a disease, TCM first determines the therapeutic principles, and then preponderance of yang belongs to excess-heat syndrome. It should be treated with cold-natured drugs in order to inhibit excessive yang, i.e, to cool the heat. Cold syndrome caused by preponderance of yin belongs to cold-excess syndrome. It should be treated with hot-natured drugs so as to restrain excessive yin, i.e., to heat the cold. As both syndromes above are excess syndromes, this therapeutic principle is called “treating excess syndromes with the purgation”. Deficiency-cold syndrome caused by yang deficiency, should be treated with the drugs warm and tonic in nature to relieve excessive yin. This is said in TCM, “restraining predominant yin by reinforcing yang,” also known as “treating yang for yin diseases.” The interior heat syndrome resulting from yin deficiency belongs to Deficiency-heat syndrome, which should be treated with the drugs of nourishing yin and replenishing fluids so as to restrict excessive yang, this is what is known in TCM, “restraining predominant yang by strengthening (renal肾) yin,” also called “treating yin for yang diseases.”

Zhang Jingyue thought that, in treating deficiency of yin or yang, drugs tonifying both yin and yang should be used, because yin and yang are interdependent. Therefore, reducing the excessive and replenish the deficient can adjust excess or deficiency between yin or yang and restore the balance between them.

The yin-yang theory is used in not only determining diagnostic principles but summarizing the nature, flavor and action of medicinal herbs. Thus providing a theoretical basis for the clinical application of herbs.

In terms of the medicinal nature, herbs with cold nature belong to yin and those with warm and hot nature pertain to yang. As concerns the flavors, herbs that are sour, bitter and salty belong to yin and those that are acrid, sweet and bland pertain to yang. In terms of actions, herbs with astringent, descending and sinking actions belong to yin, while those with dispersing, ascending and floating actions pertain to yang.

To sum up, the principle of treatment should be established in the light of the excess or deficiency of yin or yang, and then relevant(相关的) herbs should be selected according to the attribution(归因) of yin or yang and their functions. So and so only can imbalance of yin and yang be put right, and eventually, the aim of curing diseases is attained.
Special phrases 特殊词组

treating…with…

treating excess syndrome with purgation purging excess

treating deficiency syndrome with invigoration， (or invigorating deficiency)

treating cold syndrome with hot-natured drugs，( or heating the cold )

treating heat syndrome with cold-natured drugs，( or cooling the heat )