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 the yin-yang theory. So TCM bolds, "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, narmely, 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 sums up the nature of medicinal herbs. Heat syndrome caused by 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 the excess - 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) 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 herbs should be selected according to the attribution 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.

责任编辑：admin