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The opposition of yin and yang

The theory of yin-yang states that everything in nature has two opposite aspects, namely yin and yang. The opposition of yin and yang is mainly reflected in their ability to struggle with, and thus control each other. For instance, warmth and heat (yang) may dispel cold, while coolness and cold (yin) may lower a high temperature. The yin or yang aspect within any phenomenon will restrict the other through opposition. Under normal conditions in the human body, therefore, a relative physiological balance is maintained through the mutual opposition of yin and yang. If for any reason this mutual opposition results in an excess or deficiency of yin or yang, the relative physiological balance of the body will be destroyed, and disease will arise. Examples are excess of yin leading to deficiency of yang, or hyperactivity of yang leading to deficiency of yin. This is referred to in Chapter 5 of the book Plain Questions: "When yin predominates, yang will be diseased; when yang predominates, yin will be diseased."

The interdependence of yin and yang

Yin and yang oppose each other and yet, at the same time, also have a mutually dependent relationship. Neither can exist in isolation: without yin there can be no yang, without yang no yin. Without upward movement (yang) there can be no downward movement (yin). Without cold (yin) there would be no heat (yang). Both yin and yang are the condition for the other's existence, and this relationship is known as the interdependence of yin and yang. The fifth chapter of Plain Questions says, "Yin remains inside to act as a guard for yang, and yang stays outside to act as a servant for yin."

When this is applied to the physiology of the human body yin corresponds to nutrient substances, and yang to functional activitics. The nutrient substances remain in the interior, therefore "yin remains inside", while the functional activities manifest on the exterior, so "yang remains outside." The yang on the exterior is the manifestation of the substantial movement in the interior, so it is known as "the servant of yin." The yin in the interior is the material base for functional activities and is therefore called the "guard of yang." It is stated in the Chapter "Manifestations of Yin and Yang" of Illustrated Supplement to the Classified Classics: "Without yang there would be no production of yin; without yin there would be no production of yang."

The inter-consuming-supporting relationship of yin and yang

The two aspects of yin and yang within any phenomenon are not fixed, but in a state of continuous mutual consumption and support. For instance, the various functional activities (yang) of the body will necessarily consume a certain amount of nutrient substance (yin). This is the process of "consumption of yin leading to gaining of yang". On the other hand, the production of various nutrient substances (yin) will necessarily consume a certain amount of energy (yang). This is the process of "consumption of yang leading to the gaining of yin." Under normal conditions, the inter-consuming-supporting relation of yin and yang is in a state of relative balance. If this relationship goes beyond normal physiological limits, however, the relative balance of yin and yang will not be maintained, resulting in excess or deficiency of either yin or yang and the occurrence of disease.

The inter-transforming relationship of yin and yang

The two aspects of yin and yang within any phenomenon are not absolutely static. In certain circumstances, either of the two may transform into its opposite, i.e. yang may transform into yin, and yin into yang. If the inter-consuming-supporting relationship is a process of quantitative change, then the inter-transformation of yin and yang is a process of qualitative change.

The fifth chapter of Plain Questions says, "Extreme yin will necessarily produce yang, and extreme yang will necessarily produce yin. ... Severe cold will give birth to heat, and severe heat will give birth to cold."

On the one hand, this illustrates the inter-transformation of yin and yang, and on the other hand, the circumstances needed for their transformation. Without the combination of both internal and external factors, the transformation will not occur. Acute febrile disease is an example. Extreme heat severely consumes and damages the antipathogenic qi of the organism. After persistent high fever, severe cold manifestations may appear, such as a sudden drop in body temperature, pallor, cold limbs and a fading pulse. If proper emergency treatment is given in time, the yang qi will be resuscitated and there will be an improvement in the pathological condition, with the limbs becoming warm and the complexion and pulse returning to normal. The former is yang transforming into yin, and the latter yin transforming into yang.

The infinite divisibility of yin and yang

As already mentioned, yin and yang are in a state of constant change. This means that there are relative degrees of both yin and yang. It is stated in the sixth chapter of Plain Questions: "Yin and yang could amount to ten in number; they could be extended to one hundred, one thousand, ten thousand or infinity. Although infinitely divisible, yin and yang are based upon only one important principle."
According to circumstances, yin and yang can be amplified into three subdivisions respectively. Chapter 66 of the book Plain Questions says, "The qi of yin and yang may be lesser or greater. That is why there are three yin and three yang."
This quotation explains that the qi of yin and yang may be greater or lesser in degree and that there are three sub-divisions of yin and three of yang. Greater Yin is called Taiyin (the third yin), Lesser Yin is called Shaoyin (the second yin), Greater Yang is called Taiyang (the third yang), Scanty Yang is called Shaoyang (the first yang), Extreme Yang is called Yangming (the second yang) and Declining Yin is called Jueyin (the first yin). The three yin and the three yang are a further amplification of yin and yang, and also reflect the consuming- supporting relationship of yin and yang. The differentiation of syndromes applied to the development of febrile diseases is analysed with the application of the Taiyang, Yangming, Shaoyang, Taiyin, Shaoyin and Jueyin categories.

The above mentioned is the basic content of the theory of yin-yang, the cardinal principles of which are explained by the "Yin-Yang Figure" (Taijitu). In this illustration, the white colour indicates yang, and the black colour yin. The opposition and interdependence of yin and yang are illustrated by the curved line showing the inter-consuming-supporting relationship, The white yang area contains a black spot (yin) and the black yin area a white spot (yang) indicating the potential for inter- transformation, yin within yang and yang within yin. This illustration shows that all phenomena are not isolated, but inter-connected, developing and changing.

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