TCM believes the root cause of disease is imbalance between yin and yang. So the sole purpose for treating disease is to readjust yin and yang and restore the normal balance between yin and yang. Since the main manifestations of imbalance between yin and yang are relative predomination of yin and yang and relative decline of yin and yang, the purpose of regulating yin and yang is to reduce the excess and supplement deficiency.

Relative predomination of yin or yang means excess. Relative predomination of yin means excess of yin and relative predomination of yang means excess of yang. Reduce excess means to reduce relative predomination and restore the normal state of yin or yang. Since the syndrome caused by excess of yin or yang is of excess in nature and tile treatment of excess syndrome is purgation, the principle for "reducing excess" is also known as "excess should be treated by purgation" in TCM.

     

Relative predomination of yin causes excess-cold syndrome while relative predomination of yang causes excess heat syndrome. The therapeutic principle for treating excess-cold syndrome is "to treat cold syndrome with heat therapy" while the therapeutic principle for treating excess-heat syndrome is "to treat heat syndrome with cold therapy."

Since yin and yang are opposite to each other, predomination of yin may damage yang and predomination of yang may impair yin. So in the treatment of syndromes due to predomination of yin or yang, cares should be taken to differentiate the state of the other side so as to take both sides into consideration if necessary, i.e. dispersing yin-cold in combination with strengthening yang and clearing yang-heat in combination with nourishing yin.

Relative decline of yin or yang means insufficiency. Relative decline of yin means insufficiency of yin and relative decline of yang means insufficiency of yang. Supplementing insufficiency means rectifying decline and restoring the normal state of yin or yang. Since the syndrome caused by relative decline of yin or yang is of deficiency in nature, the general principle for treating deficiency syndrome is "supplementation".

Relative decline of yin causes deficiency-heat syndrome while relative decline of yang causes deficiency-cold syndrome. For the treatment of deficiency-heat syndrome, yin-nourishing therapy can be used (to use tonic herbs to nourish yin and supplement blood); for the treatment of deficiency-cold syndrome, yang-supplementing therapy can be used (to use tonic herbs of warm nature to strengthen yang and supplement qi). 

However deficiency cold syndrome due to relative decline of yang may result from retention of water in the body caused by failure of deficient yang to transform qi. In this case purging therapy cannot be used to eliminate retention of water. Instead, measures should be taken to warm and supplement yang-qi. When yang-qi is activated, qi will flow smoothly and retention of water will disperse naturally. That is why it was said in ancient times that "supplementing the source of fire can eliminate superabundance of yin."

On the other hand deficiency-heat syndrome may result from endogenous fire due to failure of deficient yin to control yang. In this case purging and clearing therapy cannot be used to eliminate fire and heat. Instead, measures should be taken to enrich yin-essence. When yin-essence is enriched, the state of yin and yang will be normalized. When yang is able to maintain latent, fire will disappear naturally. That is why it was said in ancient times that "strengthening water source can control predominant yang."

Since yin and yang depend on each other, relative decline of the one will certainly involve the other, eventually leading to decline of both yin and yang which will bring on deficiency of both yin and yang. For the treatment of such a morbid condition, both yin and yang should be supplemented. 

For this reason, simple yin deficiency syndrome can be treated by yin-nourishing therapy with the addition of herbs for strengthening yang for the purpose of promoting the transformation of yin with the qi-transforming function of yang, known as obtaining yin from yang; simple yang deficiency syndrome can be treated by yang-nourishing therapy with the addition of herbs for nourishing yin for the purpose of promoting the transformation of yang with the essence-nourishing function of yin, known as obtaining yang from yin.

     

Besides, the meridians, the viscera and the activities of ascent, descent, out-going and entering all pertain to yin and yang in nature respectively, the principles for regulating yin and yang can be used to generalize other related therapeutic methods. For example, the methods for regulating disorder of the meridians, qi and blood, zang-organs and fu-organs as well as ascent, descent, out-going and entering are all included in the concept of "regulating yin and yang."