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 Tao", (Tao 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 between 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.