The Yin Yang theory originates in ancient Chinese philosophy and metaphysics, which describes two primal opposing but complementary components based on the observation of natural phenomena. By observing the sky, ancient Chinese found that the sun is hot and bright and moon is cold and doom. The sun and moon rise and set every day around the ground, bring to people day and night. They observed the dipper and noticed the seasonal changes of spring, summer, fall, and winter. Despite these changes, they found that the day-night, seasonal and annual cycles are repeated and the rules are unchanged. Chinese philosophy uses yin and yang to represent a wider range of opposite properties in the universe: day and night, sun and moon, cold and hot, slow and fast, still and moving, masculine and feminine, lower and upper, etc. Ancient Chinese believe human is part of the universe and can be explained by universal rules. Without the modern techniques for the anatomy and pathology, they used the Yin Yang theory to explain the body of themselves and gradually developed part of TCM principles. The Yin Yang concept was thus used to direct the diagnosis and treatment.

Yin, which represents the darker element, is passive, dark, feminine, downward-seeking, and corresponds to the night; yang, which represents the brighter element, is active, light, masculine, upward-seeking and corresponds to the day. A circular representation of yin-yang (Yin is symbolized by the color black and yang in white) can summarize the basic rules:

1. Yin and Yang do not exclude each other, they are interdependent.

Yin is the opposite of yang and yang is the opposite of yin. However, no one thing is completely Yin or completely Yang. One cannot exist without the other. Each contains the seed of its opposite. For example, day is yang and night is yin. Day cannot exist without night. The end of the day is the beginning of night. Vice versa.

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2. Yin and Yang can be further subdivided into Yin and Yang. Part of Yin is in Yang and part of Yang is in Yin.

Any Yin or Yang aspect can be further subdivided into Yin and Yang. For example, the beginning is yang and the end is yin. The day is yang and the night is yin. However, the day contains the beginning and the end. So the end of the day is the yin in the yang and the beginning of night is the yang in the yin.

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3. Yin and Yang consume each other.

Yin and Yang are usually held in balance and existing in harmony: as one increases, the other decreases. However, imbalances can occur. There are four possible imbalances: excess Yin, excess Yang, Yin deficiency, and Yang deficiency. For example, when day is longer, night is shorter. When the day time is 24 hours (like the summer at the South Pole), there is no night.

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4. Yin and Yang can transform into one another.

At a particular stage, Yin can transform into Yang and vice versa. For example, night changes into day; warmth cools; life changes to death. However this transformation is relative too. Night and day coexist on Earth at the same time when shown from space.

TCM deems that human life is a physiological process in constant motion and change. Under normal conditions, the increase and decrease of yin and yang are kept within certain bounds, reflecting a delicate dynamic balance of the physiological processes. When the balance is broken, disease occurs. Typical cases of disease-related imbalance include excess of yin, excess of yang, deficiency of yin, and deficiency of yang.

During the long term process of exploit and experiments (under the leader of Shennong Emperor), ancient Chinese found the mechanism of herbs and food to strengthen yang and nourish yin, thus to harmonize and adjust these two principles in human body. They believe that food and herbal treatments can correct yin-yang imbalances by supplementing the deficient element. For example, in summer, cooling yin foods, such as vegetables and fresh fruits, should be increased in the diet, and in winter, more warming yang foods, like meats, seafood, tonics, and fries, should be added to the meal. There is an authoritative book, Zhong Hua Ben Cao, describes the properties of Chinese herbs to compensate the deficiency of yin (yang) or suppress symptoms of excessive yin (yang). Based on the herbs Yin and Yang properties, the TCM physicians designs a remedy with many herbs tailored and combined together to restore the yin yang balance for patients.