The Yin Yang theory holds that all phenomena consist of two opposite aspects, yin and yang, which are variously defined as: up and down, left and right, light and dark, hot and cold, stillness and movement, substance and function, etc. The movements and changes of yin and yang give impetus to the development of everything or in the words of the Suwen, "Yin and yang are the law of Heaven and Earth, the outline of everything, the parents of change, the origin of birth and destruction...."

Yin and yang represent two opposite aspects of every object and its implicit conflict and interdependence. Generally, anything that is moving, ascending, bright, progressing, hyperactive, including functional disease of the body, pertains to yang. The characteristics of stillness, descending, darkness, degeneration, hypoactivity, including organic disease, pertain to yin.

The nature of yin and yang is relative. According to Yin-Yang theory, everything in the universe can be divided into the two opposite but complementary aspects of yin and yang and so on ad infinitum. For example, day is yang and night is yin, but morning is understood as being yang within yang, afternoon is yin within yang, evening before midnight is yin within yin and the time after midnight is yang within yin. As the Suwen states, "Yin and yang could amount to ten in number, be extended to one hundred, to one thousand, to ten thousand and ever to the infinite." The Application of Yin-Yang Theory to the Field of Traditional Chinese Medicine

The theory of yin and yang is used extensively in traditional Chinese medicine to explain the histological structure, physiological function, and pathological changes of the human body, and to serve as guide for diagnosis of treatment.

The Anatomical and Histological Structure of the Human Body

The Yin-Yang theory asserts that the human body is an organic whole, and there exists an organic connection between all tissues and structures. Yet, at the same time, each of them can be divided into the opposite aspects of yin and yang. Viewing the body as a whole, the portion above the waist pertains to yang and that below belongs to yin; the exterior of the body is associated with yang, while the interior is associated with yin; the back is considered yang and the front, yin; the lateral aspect is yang and the medial, yin.

The zang-fu organs also have yin and yang aspects, the six fu organs are considered yang while the zang organs are yin. Each of the zang-fu organs itself can again be divided into yin or yang; for example, heart yin and heart yang or kidney yin and kidney yang. However complex, all human body structures and tissues can be generalized and explained by the yin-yang relationship. As the Suwen says, "Man has physical shape which is inseparable from yin and yang."

The Physiological Functions of the Human Body

The Yin-Yang theory considers the normal vital activities of the human body to be the result of the relative balance between yin and yang. In traditional Chinese medicine, the physiological functions of the organs and their substances are inseparably related to yin and yang. For example, the activities (yang) of a particular organ are based on that organ's substance (yin) and when either of these aspects is absent, the other cannot function. Thus the result of physiological activities is to constantly promote the transformation of yang into yin essence. If yin and yang cannot maintain relative balance and interaction, they will separate from each other ending the life that depends upon them. As the Suwen says, "When yin keeps balance with yang and both maintain a normal condition of qi, then health will be high-spirited. A separation of yin and yang will lead to the exhaustion of essential qi."

The Pathological Changes of the Human Body

The Yin-Yang theory holds that disease is a result of an imbalance between yin and yang which leads to the hyperactivity or hypoactivity of yin and yang. The occurrence and the development of a disease are also related to zheng qi (body resistance or antipathogenic factors) and xie qi (pathogenic factors). The Yin-Yang theory can be used to generalize the interacting relations between body resistance and antipathogenic factors. Pathogenic factors are divided into yang-natured pathogenic factors and yin-natured pathogenic factors, while zheng qi includes yin essence and yang qi. Yang pathogenic factors may bring about hypoactivity of bodily yang which leads to injury of yin; a heat syndrome results. If the disease is caused by yin pathogenic factors, it may give rise to hypoactivity of yin followed by the injury of yang; a cold syndrome will result. When yang is deficient it fails to restrict yin in the balanced relationship between the two giving rise to xu (deficiency) which is a cold syndrome. The xu heat symptoms complex, however, is caused by a yin deficiency and yang excess. Pathological changes of disease are varied, but can be generally explained in terms of yin-yang imbalance: yin excess causes cold syndromes, yang preponderance leads to heat syndromes, yang deficiency causes cold syndromes, and yin deficiency leads to heat syndromes.