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Coupled with the theory of the five elements, the theory of yin-yang is one part of the interpretation of natural phenomena that originated in ancient China. It reflected a primitive concept of materialism and dialectics and played an active role in promoting the development of natural science in China. Ancient physicians applied the theory to the field of medicine, profoundly influencing the formation and development of the theoretical system of traditional Chinese medicine, and guiding clinical work up to the present time.

The theory of yin-yang is a conceptual framework which was used for observing and analysing the material world in ancient China. The early theory of yin-yang was formed in the Yin and Zhou dynasties (16th century - 221 B.C.). The term yin-yang first appeared in The Book of Changes, "Yin and yang reflect all the forms and characteristics existing in the universe".

Up to the Spring and Autumn Period (770 - 476 B.C.) and the Warring States Period (475 - 221 B.C.), the application of the theory of yin-yang had been deeply rooted in all schools of thought. It was pointed out in Chapter 5 of the book Plain Questions, "Yin and yang are the laws of heaven and earth, the great framework of everything, the parents of change, the root and beginning of life and death ... ".

This quote expresses the idea that all natural events and states of being are rooted in yin and yang and can be analysed by the theory of yin-yang. The theory of yin-yang, however, does not itself refer to any concrete objective phenomena. It is, rather, a theoretical method for observing and analysing phenomena. Briefly speaking, yin and yang are a philosophical conceptualization, a means to generalize the two opposite principles which may be observed in all related phenomena within the natural world. They may represent two separate phenomena with opposing natures, as well as different and opposite aspects within the same phenomenon. Thus the ancient Chinese people, in the course of their everyday life and work, came to understand that all aspects of the natural world could be seen as having a dual aspect, for example, day and night, brightness and dimness, movement and stillness, upward and downward directions, heat and cold, etc. The terms yin and yang are applied to express these dual and opposit qualities.

Chapter 5 of the book Plain Questions states: "Water and fire are symbols of yin and yang." This means that water and fire represent the two primary opposite aspects of a contradiction based on the properties of water and fire, everything in the natural environment may be classified as either yin or yang. Those with the basic properties of fire, such as heat, movement, brightness, upward and outward directions, excitement and potency, pertain to yang. Those with the basic properties of water, such as coldness, stillness, dimness, downward and inward directions, inhibition and weakness, pertain to yin. Accordingly, within the field of medicine different functions and properties of the body are classified as either yin or yang. For example, the qi of the body which has moving and warming functions is yang, while the qi of the body which has nourishing and moistening functions is yin.

The yin-yang nature of a phenomenon is not absolute but relative. This relativity is reflected in two ways. On the one hand, under certain conditions yin may change into yang and vice versa (the inter-transforming nature of yin and yang). On the other hand, any phenomenon may be infinitely divided into its yin and yang aspects, reflecting its own inner yin-yang relationship. Day, for example is yang, while night is yin. Each, however, can be further classified as follows: morning is yang within yang, afternoon yin within yang, the first half of the night yin within yin, and the second half of the night yang within yin. This differentiation of the natural world into its opposite parts can be carried out infinitely.

It can be seen, therefore, that yin and yang are at the same time opposite in nature and yet interdependent. They both oppose and complement each other, and exist within all natural phenomena. Traditional Chinese medicine applies the yin-yang principles of interconnection and continuous transformation to the human body to explain its physiology and pathology and to guide clinical diagnosis and treatment.

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