Yin and yang are the general descriptive terms* for the two opposite, complementary* and inter-related cosmic forces* found in all matter in nature. The ceaseless motion of both yin and yang gives rise to all changes seen in the world.

Yin, in Chinese philosophy, refers to the feminine*, latent* and passive principle* (characterized by dark, cold, wetness, passivity, disintegration*, etc.), while yang refers to the masculine*, active and positive principle (characterized by light, warmth, dryness, activity, creation, etc.). These are the two opposing cosmic forces (yin and yang) into which creative energy is divided and whose fusion in physical matter brings the phenomenal world into being.

The theory shows that the world is a material whole and all the materials in the universe contain two opposite aspects, such as the sun and the moon, day and night, cold and hot, water and fire (of nature), clear and cloudy (of the sky), man and woman (of mankind).

Opposition of yin and yang* refers to the mutually opposing, repelling* and contending* relationship between yin and yang. For example, warmth and heat are yang and may dispel cold, while coolness and cold are yin and may lower a high temperature. Mutual rooting of yin and yang* refers to the mutually dependent relationship between yin and yang. Without yin, there is no yang; and without yang, there is no yin. Waxing and waning of yin and yang* refers to changes in the strength and prevalence* of the pared yin and yang. Yin and Yang have an inter supporting relationship.

In some cases, yin increases when yang decreases or yin decreases when yang increases. For instance, from winter to spring and to summer, the weather gradually becomes warm and hot; this is the process in which yang increases while yin decreases. From summer to autumn and to winter, the weather gradually changes to cool and cold, and this is the process of yin increasing while yang decreases.

In other cases, yin and yang increase and decrease at the same time. Take human growth as an example. After birth, from infancy to the prime of life, the physical body (which pertains to yin) and the person's functional ability (which pertains to yang) grow together. As a result, the infant grows into an adult, the body becomes big and the functional ability becomes strong.

Yin-yang harmony* refers to the state in which yin and yang are in harmonious coordination. When yang functional activities and yin nutritional substances enrich* each other harmoniously, the body remains strong and mental activities are normal.

Yin-yang conversion* means the property of the same thing can be transformed between yin and yang. The reason for yin to be able to convert into yang and yang to be able to convert into yin is that their existence relies on each other and they contain each other. Yin contains some yang and yang contains some yin; when yin within yang increases to a certain degree, the whole thing changes into yin; when yang within yin increases to a certain degree, the whole thing changes into yang.

Balance and harmony of yin and yang in the human body are the essential conditions for the maintenance of normal life activities; when yin and yang are in disharmony or imbalance, disease occurs and develops.

When yin pathogen* causes disease, body yang is likely to be damaged; when yang pathogen* causes disease, body yin is likely to be damaged. Fig. 2-3 lists the common pathological conditions of yin and yang out of balance.

Yin or yang exuberance, i. e. exuberance of yin or exuberance of yang is a morbid* state in which either yin or yang is higher than its normal level. The condition leads to excess syndrome, i. e. excess cold syndrome* caused by exuberance of yin and excess heat syndrome* caused by exuberance of yang. Yin exuberance is a pathological state in which yin pathogen is exuberant while the healthy qi has not been damaged, giving rise to an excess cold syndrome. Yang exuberance is a pathological state in which yang is exuberant while yin has not been debilitated, giving rise to an excess heat syndrome. Yin or yang deficiency is a morbid state in which either yin or yang is lower than its normal level. This condition leads to deficiency syndrome, i. e. deficiency heat syndrome* caused by deficiency of yin and deficiency cold syndrome* caused by deficiency of yang.

Yin deficiency is a pathological state marked by deficiency of yin with diminished moistening, calming, down bearing* and yin-inhibiting* function, leading to* relative hyperactivity of yang.* Yang deficiency is a pathological state characterized by deficiency of body's yang qi that leads to diminished functions, decreased metabolic activities, reduced body reactions as well as deficiency cold manifestations*. Yin and yang are mutually rooted; they are interdependent and inter-complementary. Detriment of yin affects yang* and detriment of yang affects yin*, which results in both yin and yang deficiency.

Yin-yang conversion refers to the so-called inter-transformation of yin and yang. Yin-yang properties of a certain syndrome, under certain conditions, can change into its opposite, that is a yin syndrome can transform into a yang syndrome and vice versa*. * Yin-yang repulsion refers to a serious pathological state in which extremely excessive yin in the interior forces the deficient yang to spread outward, or extremely exuberant yang in the interior keeps insufficient yin on the outside, forming pseudo-heat* or pseudo-cold phenomena.

Since the yin-yang disharmony or imbalance is the fundamental reason for the occurrence and development of disease, most diseases, no matter how complicated and changeable their clinical manifestations* are, can be generalized and explained with the theory of yin and yang. Yin-yang disharmony (out of balance) is one of the basic mechanisms* of disease; consequently, reestablishing a state of relative balance of yin and yang is one of the basic therapeutic * principles.