The Yin-Yang Theory

The theory of yin-yang posits that every object or phenomenon in the universe consists of two opposing aspects, namely yin and yang, which are both opposite and complementary. This relationship between yin and yang is the universal law of the material world, the principle and source of the existence of all beings, and the fundamental cause of the development and decay of things. The theory of yin-yang mainly expounds the opposition, complementarity, mutual destruction and support relationships, and the transformation relationships of yin and yang. These relationships between yin and yang are widely used in traditional Chinese medicine to explain the physiology and pathology of the human body and to guide diagnosis and treatment in clinical work.

1. The Opposition and Complementarity of Yin and Yang

The opposition of yin and yang generalizes the contradiction and struggle of two opposites within an object or phenomenon. Ancient Chinese used water and fire to symbolize the basic properties of yin and yang: the basic properties of yin resemble those of water, including coldness, a tendency to go downward, darkness, etc., while the basic properties of yang are like those of fire, including heat, a tendency to go upward, brightness, etc. From this, it can be inferred that anything possessing the characteristics of stillness, coldness, inferior position (or downward), interiority (or inward direction), dimness, asthenia, inhibition, slowness, or material substance relates to yin, while anything that is moving, hot, in a superior position (or moving upward), in an exterior position (or outward), bright, dynamic, excited, fast, energetic belongs to yang.

Since the yin or yang nature of a thing exists only by comparison and a thing can be infinitely divided, the yin or yang nature is by no means absolute, but relative. Under certain circumstances, the two opposites of a thing can change and the yin-yang nature of the thing changes accordingly.

The tissues and organs of the human body can be of yin or yang nature depending on their relative positions and functions. Considering the body as a whole, the surface of the trunk and the four limbs being external relate to yang, while the organs and viscera (zang-fu) are inside the body and are yin. Considering only the surface of the body and the four extremities, the back is yang, while the chest and abdomen are of yin nature. The parts above the waist are yang and those below are yin. The lateral side of the extremities relates to yang and the medial side to yin. The meridians that run along the lateral side of an extremity are yang, while those along the medial side are yin. When speaking of the organs and viscera (zang-fu) alone, the fu organs with their main functions of transmitting and digesting food relate to yang, while the zang organs with their main function of storing vital essence and vital energy relate to yin. Each of the zang-fu organs can again be divided into yin or yang: for example, the yin and yang of the kidney, the yin and yang of the stomach, etc. In short, all tissues and structures of the human body and their functions can be generalized and explained by the relationship of yin and yang.

The complementary relationship of yin and yang means that each is the condition for the existence of the other and neither can exist in isolation. For example, without day, there would be no night; without excitation, there would be no inhibition. Therefore, it can be seen that yin and yang are both in opposition and complementarity; they rely on each other to exist within a single entity. The movement and change of a thing are not due solely to the opposition between yin and yang but also to their relationship of interdependence and mutual support.

In physiological activities, the transformation of substances attests to the theory of the complementarity of yin and yang. Substance is yin and function is yang; the former is the basis of the latter, while the latter is the reflection of the existence of the former and also the driving force for the production of substance. There must be sufficient nutritive substances for the functional activities of the organs to be healthy. Similarly, the functional activities of the organs must be robust to stimulate the production of nutritive substances. The coordination and balance between substance and function are the essential guarantee of physiological activities. Thus, we find in the Neijing: "Yin is installed inside as the material basis of yang, while yang stays outside as the manifestation of the function of yin."

2. The Relationships of Mutual Destruction and Support and the Transformation Relationships of Yin and Yang

Destruction implies loss or weakening. Support implies gaining or strengthening. The two aspects of yin and yang within a thing are not fixed but in a state of constant movement. Due to their mutual conflict and support, the loss or gain of one aspect will inevitably influence the other. For example, the consumption of yin leads to gaining yang and vice versa. On the other hand, gaining yin leads to consuming yang and vice versa. The functional activities of the human body require a certain amount of nutritive substances, leading to the consumption of yin to gain yang, while the formation and storage of nutritive substances depend on functional activities and weaken functional energy to a certain degree, which causes a process of gaining yin and consuming yang. But this rebalancing is not systematic. Under normal conditions, these oppositions maintain a relative balance, while under abnormal conditions, excesses or deficiencies occur.

In the process of mutual destruction and support, a loss of relative balance between yin and yang causes a preponderance or deficiency of one or the other. This is the cause of disease. For example, an excess of yin will destroy yang and a deficiency of yang will result in a preponderance of yin, which can in both cases result in a cold syndrome. On the other hand, an excess of yang will consume yin and a deficiency of yin will cause an excess of yang, which can in both cases result in a heat syndrome. However, cold or heat syndromes caused by a preponderance of harmful factors relate to excess (shi), while those of cold or heat due to the lowering of general resistance pertain to deficiency (xu). These two types of syndromes are of different natures and the principles of treatment are consequently also different: dispersion (xie) for shi (excess) syndromes and tonification (bu) for xu (deficiency) syndromes.

Since the onset of a disease is the result of an imbalance between yin and yang, all treatment methods must aim to reconcile the two and restore a state of relative balance. In acupuncture treatment, points on the right side can be selected to treat disorders of the left part and vice versa, while points on the lower part of the body can be selected to treat disorders of the upper part and vice versa. All these methods are based on the concept that considers the body as an organic whole. The purpose of treatment is to readjust the relationship between yin and yang and promote the circulation of qi (energy) and blood.

The transformation of yin and yang means that under certain circumstances and at a certain stage of development, each of the two aspects of yin and yang within a thing will transform into its opposite. This transformation occurs if there is the possibility of transformation within the thing itself. Given this possibility, external conditions are also indispensable.

The development and change of a thing require a process, as well as external conditions for the gradual transformation of the thing in question. The transformation between yin and yang follows this rule. According to the Neijing: "Tranquility comes after excessive movement: extreme yang will become yin." And, "The generation of a thing is due to transformation; the degeneration of a thing is due to transmutation." This is precisely the meaning of the old adage: "When a certain limit is reached, a change in the opposite direction is inevitable." Quantitative changes lead to qualitative change.

The transformation between yin and yang is the universal law governing the development and change of things. The alternation of the four seasons is an example. Spring, with its warmth, sets in when cold winter reaches its peak, and the coolness of autumn arrives when the summer heat reaches its climax. The change in the nature of a disease is another example. A patient with a high and persistent fever in an acute febrile disease may have a lowering of body temperature with pallor and coldness of the extremities and a weak and thready pulse, indicating that the nature of the disease has changed from yang to yin. For this patient, the treatment method will need to be modified accordingly.

The above is a brief introduction to the theory of yin and yang, with some examples to illustrate its application in traditional Chinese medicine. In short, the relationships of opposition, complementarity, mutual destruction and support, and transformation of yin and yang can be summarized as the law of the unity of opposites. Furthermore, these four relationships between yin and yang are not isolated from each other but interconnected, influencing each other, and each being the cause or effect of the others.