What is Yin-Yang? Yin-Yang reflect all the forms and characteristics existing in the universe. They are the laws of heaven and earth. At their origin, these terms were used to describe the two sides of a mountain. The sunny side was Yang and the shady side was Yin.

All events of nature and states of being are rooted in Yin and Yang, and can be analyzed by the theory of Yin-Yang. The theory of Yin-Yang, however, does not itself refer to any concrete objective phenomenon. It is a theoretical method for observing and analyzing phenomena. Generally speaking, Yin and Yang are a philosophical conceptualization, a means to generalize two opposite principles that 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. The natural world could be seen as having a dual aspect, for example, day and night, brightness and dimness, movement and stillness, heat and cold, etc.

"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 Yin and Yang. Those with the basic properties of fire, such as heat, movement, brightness, upward and outward direction, excitement and potency, pertain to Yang; those with the basic properties of water, such as coldness, stillness, dimness, downward and inward direction, inhibition and weakness, pertain to Yin.

Accordingly, within the field of medicine different parts of the body are classified as either Yin or Yang. For example, the upper and exterior parts of the body belong to Yang; the lower and interior parts belong to Yin; the hands belong to Yang, while the feet belong to Yin; the five Zang organs pertain to Yin, the six Fu organs to Yang. 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 one hand, under certain conditions Yin may change into Yang and vice versa (the inter-transforming nature of Yin and Yang), and on the other hand, any phenomenon may be infinitely divided into it's Yin and Yang aspects, reflecting it's own inner Yin-Yang relationship. Day, for example, is Yang, while night is Yin. However, each can be further classified as follows: morning is yang within yang, afternoon is yin within yang, the first half of the night is yin within yin and the second half of the night is yang within yin. This differentiation of the natural world into it's opposite parts can be carried out infinitely.

Therefore it can be seen 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 it's physiology and pathology and to guide clinical diagnosis and treatment.