The Theory of Essential Qi
The Theory of Yin-Yang
I. The Major Aspects of Yin-Yang Theory
1. Opposition of yin and yang
2. Interdependence of yin and yang
3. Inter-consuming supporting relationship of yin and yang
4. Inter-transformation of yin and yang
5. Infinite divisibility of yin and yang
II. The Applications of Yin-Yang Theory in Medicine
1. Yin-yang and organic structure
2. Yin-yang and physiological functions
3. Yin-yang and pathological changes
4. Yin-yang and diagnosis
5. Yin-yang and treatment
The Theory of the Five Elements
I. Classification of Things According to the Five Elements
II. Correlation of the five Elements
III. The Application of the Five Elements Theory to Medicine


Traditional Chinese medicine is a brilliant component of Chinese culture. Characterized by a unique theoretical system and enriched by thousands of years of practical experience, it still possesses great vitality. This seems to be a miracle in the world history of medicine. How could this happen? There are many reasons; the crucial one is the dialectical viewpoint of ancient Chinese philosophical thought, which provided the theoretical basis for the founding and developing of traditional Chinese medicine. The achievements of traditional Chinese medicine, in turn, further stimulated philosophical thought, thus forming a benign circle. In fact, traditional Chinese medicine is not a collection of piecemeal experiences. Most experiences have been summed up, forming a theoretic system, both medical and philosophical. The philosophical strands that gave the greatest impetus to the development of traditional Chinese medicine are the theories of Essential Qi, Yin-Yang and the Five Elements.

The Theory of Essential Qi

Originally, qi was not a philosophical concept. It meant air, as it is commonly used in the everyday Chinese language. Its meaning was extended to the philosophical field, serving as a symbol for the primordial substance that was thought to constitute the universe. It was believed to be invisible, taking the form of particles, constantly moving and changing, and giving rise to energy and activities. According to ancient Chinese philosophy, the universe originates from Taiji, a synonym for original or primitive qi. Its essential part, or essential qi, was thought to be the basic element out of which the universe is composed, and everything in the world is produced through the movement and changes of qi.

It is stated in The Medical Classic of the Yellow Emperor: "Human life originates in the qi of Heaven and Earth, and develops according to the normal order of the four seasons." That is to say, human life is also endowed with the qi of nature. Qi is the material basis on which life activities are maintained. Various actions and changes of qi accompanied by transformation of energy are the fundamental characteristics of life activities. Without the actions and changes of qi there would be no life activities.

It is important to note that, in Chinese medicine, all life activities, including mental activities such as thought, will, and emotions, are based on the actions and changes of qi. Since qi is material, the theory of qi is a kind of materialism which explains mental activities in terms of objective matter. In addition, human beings are derived from the qi of nature, so the material world is primary, existing independent of the mind, while human beings are merely a part of nature. Based on this view, Chinese medicine emphasizes that all physiological activities are in conformity with the changes of the natural environment, and so, to preserve health one must keep in harmony with the law of nature.

Since qi is invisible and acts as the motive force for all kinds of life activities, the word qi in Chinese medicine is often translated into English as "vital energy". This, however, only refers to qi in its narrow (physiological) sense, and the world "energy" is comprehended from the viewpoint of modern physics, i.e., matter and energy are regarded as equivalents, mutually convertible.

Actually, qi in Chinese medicine can be classified into two main categories: the genuine or normal qi and the evil or pathogenic qi. The genuine qi of human beings comes from three sources: primordial qi, qi of food essence and air. Primordial qi is inborn, inherited from parents, while that qi of food essence and air are acquired. Genuine qi includes all the materials and energy essential for life activities as well as the body's resistance against disease.

Evil qi is harmful to health. It includes various pathogenic agents or factors, for example, abnormal atmospheric changes such as wind, cold, heat, dampness and dryness are all considered evil qi, if they happen in an untimely fashion or in excess.

In this context, disease is defined as a process of struggle between the genuine qi and evil qi. Between these two kinds of qi, emphasis is often put on the former, as is stated in The Medical Classic of the Yellow Emperor: "If genuine qi prevails, attack of evil qi never avails."

Regarding the classification of genuine qi and the clinical significance of the various part of genuine qi, a detailed discussion will be found in Chapter II.

The Theory of Yin-Yang

Another philosophical view in traditional Chinese medicine is dialectics. It is expressed as the theory of yin-yang. Yin and Yang were and still are two topographical terms designating the shady and sunny sides of a hill, respectively. Since everything under the sun has two sides, the shady and the sunny, by extension, yin and yang represent two opposites of an object or phenomenon. According to this theory, all things and phenomena in the world contain two opposite aspects: yin and yang, which are in conflict and at the same time mutually dependent. Therefore, this theory can be taken as a law of unity of opposites.

The theory of yin-yang as a dialectical way of thinking can be applied to any field of medicine. There is no conflict between the theory of qi and that of yin-yang. In fact, qi, as the basic element that constitutes the world, can also be divided into two, i.e., yin qi and yang qi. Yin qi chiefly refers to the material aspect of the element, and yang qi, the dynamic aspect. The Medical Classic of the Yellow Emperor states, "The clear yang qi ascends to form Heaven, while the turbid yin qi descends to form Earth."

The quintessence of the theory of yin-yang is the law of unity of opposites, which means keeping a dynamic balance. In fact, this law can also be applied to modern physiology at various levels -- at the levels of systems, organs, cells, and even molecules. For example, in the nervous system, excitation and inhibition, the sympathetic and parasympathetic functions, the actions of acetylcholine and norepinephrine are all in opposition, and between each pair of opposites a dynamic balance should be maintained for normal nervous activities. In the endocrine system, there are estrogen and testosterone in opposition; in the kidneys there are diuresis and anti-diuresis, renal tubular secretion and reabsorption; for muscles there are contraction and relaxation; for body temperature there are thermogenesis and thermolysis; for glucose metabolism there are insulin and glucagons; for blood there are coagulation and anticoagulation; in the body fluids there are acid and base; and so on. All these opposites should be kept in dynamic balance. Therefore, it can be concluded that the maintenance of dynamic balance by uniting various opposites is a general law of life activities.

Modern physiology has shown that homeostasis, metabolism and adaptability are the three characteristic features of life activities. In Chinese medicine, these three features are all well discussed, but expressed in different ways: the dynamic balance of yin-yang within the body for homeostasis, waxing and waning of yin-yang or transformation of yin-yang for metabolism, and adaptable conformity between the human being and the external environment for adaptability. If the yin-yang theory is compared with modern physiology in this way, it will be easy to find the rationale of this ancient theory from a modern perspective. Of course, there are great differences. The main difference is that the Chinese theory is macrocosmic, while the modern Western theory is microcosmic. The latter has many advantages. The deeper it goes, the more detail and precision attaches to the knowledge. However, it may have the tendency to pay more attention to the local part than to the whole body. The knowledge gained by macrocosmic observation in Chinese medicine is often general and vague, but because of thousands of years of accumulation of experience, there is a wealth of useful knowledge, which is neglected by Western medicine and its microcosmic point of view. That is why integration of the advantages of Chinese medicine with Western medicine may promote the development of world medicine as a whole.

I. The Major Aspects of Yin-Yang Theory

1. Opposition of yin and yang

The theory of yin-yang holds that everything in the world has two opposite aspects, namely, yin and yang. The yin or yang aspect within any phenomenon will restrict the other through opposition. Take the human metabolism as an example. Catabolism (destructive metabolism) and anabolism (constructive metabolism) are the two aspects of metabolism. The former may pertain to yang, while the latter to yin. They are apparently in opposition, but they should be kept in a dynamic balance so as to guarantee the normal metabolism.

2. Interdependence of yin and yang

Yin and yang oppose each other, but at the same time they are mutually dependent. Without anabolism there would be no catabolism, and without catabolism there would be no anabolism. In other words, if no living tissue could be formed through anabolism, the human body would cease to exist, and if no energy could be changed from the living tissue, the human body would die. Therefore, in the theory of yin-yang, it is said that yang exists with yin as its prerequisite, and yin exists with yang as its prerequisite.

3. Inter-consuming supporting relationship of yin and yang (waxing-warning of yin and yang)

The balance between yin and yang is not static, but dynamic. Anabolism and catabolism do not always proceed at the same speed or to the same degree. During daytime activities there is more catabolism than anabolism, while during rest or sleep at night there is more anabolism than catabolism. That is to say, there is an increase of materials at night along with a decrease of activities, and there is an increase of activities in the daytime accompanied by a decrease of its material basis. Of the two opposites of a single entity, increase or excess of the one is usually associated with decrease or deficiency of the other. As stated in The Medical Classic of the Yellow Emperor, "Consumption of yin leads to gaining of yang" and "consumption of yang leads to gaining of yin," and the whole process is called "waxing-waning of yin and yang" for short. In normal conditions, the waxing-waning of yin and yang is in a state of relative balance. If it goes beyond physiological limits, the yin-yang balance will be impaired, resulting in disease.

4. Inter-transformation of yin and yang

During metabolism there is inter-transformation of the living tissue for construction and energy for functional activities. Normally, the inter-transformation takes place within a limited range, forming a dynamic balance. However, in certain circumstances, either yin or yang may be transformed into its opposite. In an acute febrile disease, there is abnormally increased catabolism to produce a high fever. After persistent high fever, exhaustion may occur, with a sudden drop in body temperature, pallor, cold limbs and profuse sweating. This is a common example to illustrate transformation of yang (excess of energy with a heat syndrome) into yin (exhaustion of energy with a cold syndrome).

5. Infinite divisibility of yin and yang

The divisibility of yin and yang is infinite. As stated in The Medical Classic of the Yellow Emperor, "Yin and yang can be counted in tens, and can be extended to hundreds, thousands or infinity." For example, day and night can be divided into yin and yang, in which day is yang, while night is yin. Each can be further divided into yin and yang: morning is yang in yang, while afternoon is yin in yang; the period from nightfall to midnight is yin in yin, while the period of the small hours is yang in yin. If we take metabolism as an example, the divisibility of yin and yang is more explicit. Either catabolism or anabolism can be divided into opposite processes, and each process can be further divided into opposite aspects. Nutrients (yin) and energy (yang) are a pair of opposites. A part of energy released in catabolism turns into heat for maintaining the body temperature (yang in yang), and the rest is stored in adenosine triphosphate (ATP) (yin in yang). The energy released from ATP is further divided into two parts: one is used for muscular movement as mechanical energy (pertaining to yang), and the other is stored in creatine phosphate (pertaining to yin).

II. The Applications of Yin-Yang Theory in Medicine

The theory of yin-yang as a dialectic way of thinking can be applied to any field of medicine. All the life activities, including physiological and pathological processes, can be explained by opposition and transformation of yin and yang. Dynamic balance of yin and yang reflects normal physiological activities, while breakdown of the yin-yang balance leads to disease. Any treatment is aimed at restoring the normal dynamic balance of yin and yang. As Zhang Jingyue stated in his well-known The Complete Works of Jingyue (1624): "Although medicine is complicated, it can be summed up in one word, namely, yin-yang."

1. Yin-yang and organic structure

When the theory of yin-yang is applied to explain the organic structure of the human body, the underlying premise is that the human body should be viewed as an organic whole. As the whole body is involved, the exterior portion pertains to yang, while the interior to yin; the back or dorsal aspect pertains to yang, while the front or anterior aspect to yin; the upper portion pertains to yang, while the lower portion to yin. Among the zang-fu organs, zang-organs pertain to yin, while fu-organs to yang. Among the zang-organs, the Heart and Lungs, situated in the upper portion of the body (thoracic cavity), pertain to yang, while the Liver, Spleen and Kidneys, situated in the lower portion of the body (abdominal cavity), to yin. In terms of each organ, it can be further divided into yin and yang, e.g., the Heart can be divided into the Heart yin and the Heart yang, the Kidneys can be divided into the Kidney yin and the Kidney yang. (Yin and yang of a zang-fu organ denote the material and functional aspects of that organ, respectively. In this context, yin refers to the essential materials such as vital essence, nutrients, fluids an blood, and yang includes various functional activities as well as heat energy.)

2. Yin-yang and physiological functions

All the normal life activities are based on the dynamic balance between yin and yang, which oppose each other and at the same time unite in coordination. As far as function and matter are concerned, function pertains to yang, while matter pertains to yin. All physiological activities have their material basis; without material movements, there would be no physiological functions. On the other hand, physiological activities, in turn, promote material metabolism. Therefore, the interrelationship of unction and matter is precisely the interdependent and inter-consuming supporting relationship of yin and yang.

3. Yin-yang and pathological changes

The dynamic balance of yin and yang guarantees health. Once the balance is impaired, disease ensues. Breakdown of the normal balance of yin and yang is related to both pathogenic and anti-pathogenic factors. There are two types of pathogenic factors: yin pathogenic factors and yang pathogenic factors. For example, cold and damp are yin pathogenic factors, while wind, dryness, heat and fire are yang pathogenic factors. Anti-pathogenic factors also include yin and yang, i.e., the material aspect and the functional aspect. When yin pathogenic factors cause disease, this may lead to preponderance of yin, which damages yang and gives rise to cold syndromes. When yang pathogenic factors cause disease, this may lead to an excess of yang, which gives rise to heat syndromes and consumes yin. (As stated in The Medical Classic of the Yellow Emperor, "Yin in excess consumes yang, and yang in excess consumes yin." "Yin in excess causes cold syndromes, and yang in excess caused heat syndromes.")

If the disease is due to deficiency of the anti-pathogenic factors, there are deficiency syndromes. According to the principle of equilibrium between yin and yang, deficiency of yin leads to relative preponderance of yang, giving rise to internal heat, while deficiency of yang leads to relative preponderance of yin, giving rise to external cold.

In advanced cases, however, deficiency of either yin or yang may lead to the consumption of the other, known respectively as "impairment of yin affecting yang" and "impairment of yang affecting yin."

4. Yin-yang and diagnosis

Since diseases or syndromes can be classified into yin and yang according to their nature, and the root cause of the occurrence and development of disease is imbalance between yin and yang, the key to clinical diagnosis is correct differentiation of the yin and yang nature of the disease as well as determination of the imbalanced condition of yin and yang. In this way, complicated clinical conditions can be simplified, and a correct diagnosis can be made.

5. Yin-yang and treatment

Treatment in Chinese medicine is always aimed at restoring the normal balance of yin-yang. The following therapeutic principles derived from the theory of yin-yang are of the utmost significance.

"Replenish what is in deficiency," "reduce what is in excess," "treat cold with warming measures" and "treat heat with cooling measures" are the general rules of treatment in order to restore the normal balance of yin-yang.

An acute disease due to invasion of exogenous pathogenic factors is usually excess in nature, if the patient is normally healthy. In this case, the treatment is to eliminate or reduce the pathogenic factors. For pathogenic factors of a yin nature (such as cold), yang measures (e.g., warming therapy) should be used, and for pathogenic factors of a yang nature (such as heat), yin measures (e.g., cooling therapy) are appropriate.

In deficiency syndromes, replenishing or reinforcing measures are indicated. Deficiency of yin is usually accompanied by relative preponderance of yang, and deficiency of yang by relative preponderance of yin, and the treatment should be to replenish yin and reinforce yang, respectively. When the deficiency is put right, the relative preponderance will naturally disappear. In complicated cases, however, the treatment may also be complex. Sometimes it is necessary to "treat the yang aspect for diseases of a yin nature," and "treat the yin aspect for diseases of a yang nature," for example, by reducing yang to relieve dizziness and headaches in hypertensive patients suffering from yin deficiency with relative exuberance of yang, and replenish yin-fluid in cases of high fever (yang-heat) which consumes the body fluids. In acupuncture, it is not unusual to needle the points of the yang meridians for disorders of the yin meridians, and vice versa.

The Theory of the Five Elements

The theory of the five elements also belongs to philosophical materialism. It holds that the natural world is made up by five elements, namely, wood, fire, earth, metal and water. The ancient Chinese recognized that each element had its own properties: wood (trees) tends to spread out freely; metal is the material for manufacturing weapons; fire tends to flare upwards; earth produces myriads of things; and water tends to flow downward. By extension of the doctrine, all things can also be classified into five categories according to their properties and actions, each category pertaining to one of the five elements. In addition, there are various interrelationships among the five elements, so that they are all linked together, making the world a unified entity. Applied to medicine, a theoretic system of physiology and pathology was thus formed with the five zang-organs, namely, the Liver, Heart, Spleen, Lungs and Kidneys, as the core. The correlation among the zang-organs and between zang-organs and other organs and tissues as well as various physiological and pathological phenomena could all be explained with reference to the interrelationships of the five elements.

I. Classification of Things According to the Five Elements

In order to categorize physiological phenomena in a systematic way, by using analogy and deduction, the ancient Chinese put them into five categories, each having properties or actions similar to one of the five elements. For example, the Liver corresponds to wood because it promotes the spreading of qi and blood like a tree spreading out freely. Trees produce green leaves, so green corresponds to wood. Immature fruit, green in color, is usually sour, and so sourness is related to wood. Trees germinate in spring, and the weather in spring is neither too hot nor too cold, but is often windy. So spring is the season pertaining to wood, and wind is also related to wood. The eyes and tendons all pertain to wood, because the condition of the Liver can be reflected in the eyes, and the Liver controls the tendons. Anger is apt to impair the Liver, and patients with liver troubles are usually irascible. Therefore, among the various emotions, anger is classified into the category of wood. Such classifications are listed in the following table.

Nature					Human Body
Taste	Color	Atmos. factor	Season	Element	Zang- organs	Fu-organs	Sense organs	Tissue	Emotion
Sour	Green (Blue)	Wind	Spring	Wood	Liver	Gallbladder	Eye	Tendon	Anger
Bitter	Red	Heat	Summer	Fire	Heart	Small Intestine	Tongue	Vessel	Joy
Sweet	Yellow	Damp	Late Summer	Earth	Spleen	Stomach	Mouth	Flesh	Worry
Pungent	White	Dryness	Autumn	Metal	Lung	Large intestine	Nose	Skin & Hair	Sadness
Salty	Black	Cold	Winter	Water	Kidney	Bladder	Ear	Bone	Fear

                    
It is believed that there must be close relationships among the things and phenomena classified in the same category. Sour taste, green color, windy weather, and the season of spring, and the Liver, Gallbladder, eyes and tendons of the human body all correspond to wood, so they should be closely related to each other. The same applies to the other four categories. Therefore, the classification of things and phenomena provides a basis for the conformity of man with nature.

II. Correlation of the five Elements

According to the five elements theory, correlation is not confined o things and phenomena of the same category of element, it also exists among those of different categories. The interrelationships of the five elements include inter-promoting (producing), interacting (checking), overriding and counteracting. The promoting or producing sequence is: wood → fire → earth → metal → water → wood; the acting upon or checking sequence is: wood → earth → water → fire → metal → wood; the overriding sequence is the same as the acting upon sequence; the counteracting sequence is opposite to the acting upon sequence, i.e., wood → metal → fire → water → earth → wood.

Inter-promoting and interacting are two inseparable aspects of the five elements which both oppose and cooperate with each other, thus forming a relative and dynamic balance and coordination during the development and change of any event or thing. Therefore, each element plays a role of "promoting" (or "producing") and at the same time "being promoted" (or "being produced"), and also role of "checking" and at the same time "being checked." For example, wood promotes fire, and is promoted by water; it checks earth, but is checked by metal. The inter-promoting relationship of the five elements is also known as the "mother-child" relationship, with each element being the "child" of the element that produces it, and the "mother" of the one it produces. For example, wood is the "mother" of fire, and is the "child" of water.

III. The Application of the Five Elements Theory to Medicine

Applied to Chinese medicine, the interrelationships of the five elements further explain the correlation between the various parts of the human body as well as the correlation between the human body and the natural environment. They are particularly useful for explaining the pathogenesis of complicated physiological and pathological conditions. Following are some examples:

Water nourishes wood. This figuratively denotes that a sound Liver needs adequate nourishment of Kidney yin. If Kidney yin is insufficient, usually there is a deficiency of Liver yin accompanied by exuberant Liver yang, manifested by dizziness, tinnitus and blurred vision. In the latter case, the pathogenesis is attributed to "failure of water to nourish wood," and the treatment is "replenishing water to nourish wood," i.e., replenishing Kidney yin as the major therapeutic measure.

Wood acts on earth. This explains the physiological relationship between the Liver (wood) and the Spleen and Stomach (earth). However, if the Liver is diseased (e.g., depressed or stagnated), it may act on the Spleen and Stomach excessively, causing dysfunction of the latter. This is called the "overriding by wood of earth."

Earth promotes metal. Physiologically the Spleen (earth) transforms the nutrients from food and transports them to the Lungs (metal). This rule can also be used in the treatment of a chronic consumptive disease of the Lungs. In treating such a disease, reinforcement of the Spleen usually plays an important role. This is called "reinforcing earth to strengthen metal."

So far as the "mother-child" relationship is concerned, if the Liver is the "mother" organ, then the Heart is the "child" organ. Deficiency of Heart blood may involve the Liver, causing blood deficiency of both the Heart and Liver. Hyperactivity of the Heart may induce hyperactivity of the Liver, resulting in hyperactivity of both the Heart and Liver. Both of the above examples illustrate the conditions expressed as "a diseased child organ implicates its mother organ."

Closely related to the "mother-child" relationship is the following principle of treatment, which is particularly useful in acupuncture therapy. "Reinforce the 'mother' in deficiency conditions, and reduce the 'child' in excess conditions." This principle is based on the inter-promoting relationship among the five elements. During acupuncture, if the syndrome is deficient in nature, points of the mother meridian are usually selected for reinforcement. For example, in a patient with deficiency of Liver yin, reinforcing manipulation at Yingu (KI 10) is often applied, because Yingu is the sea point of the Kidney Meridian. If the syndrome is excess in nature, points of the child meridian are usually selected for reduction. For example, in a patient with exuberant Liver fire, reducing manipulation at Shaofu (HT 8) is often helpful, because Shaofu is the spring point of the Heart Meridian. Of course, in both examples, the points of the Liver Meridian itself can also be selected.

As Chinese medicine developed and became more sophisticated, the original rules and sequences of interrelationships among the five elements were felt to be no longer adequate to explain all the physiological and pathological changes. Therefore, some modifications were made to the five elements theory. For instance, the descending actions of the Lungs (to send down essence and qi, to control the passage of water, etc.) are helpful to the Kidneys, but the latter also assist the Lungs in inspiration. (Insufficiency of the Kidney function may lead to dyspnea characterized by shortened inspiration and prolonged expiration.) According to the five elements theory, we just say "metal promotes water," but clinically we usually say "metal and water promote each other."

Another problem is terminology. Water and fire are two of the five elements, but they are also used in etiology as pathogenic factors, and in diagnosis as names of syndromes. Particularly, the word "fire" is very misleading. Even in physiological conditions, fire, i.e., heat energy necessary for all life activities, particularly for the function of Spleen in digestion, absorption and assimilation, is produced by the Kidneys. It is called "vital fire," an equivalent to "Kidney yang." Although fire (Heart) promotes earth (Spleen) through blood circulation, but by saying "fire promotes earth," we usually mean that the vital fire warms up the digestive function of the Spleen. In chronic diarrhea accompanied by edema and intolerance of cold, the diagnosis is "fire fails to promote earth," and the treatment is "reinforce fire to promote earth." In these statements the word "fire" refers to vital fire, i.e., Kidney yang. In some other instances, however, fire still denotes the Heart. "Coordination of water and fire" explains the physiological interrelationship between the Kidneys and the Heart. If Kidney yin (water) is insufficient to check the activity of the Heart (fire), there may occur "incoordination of water and fire," marked by restlessness and insomnia.

The confusion in terminology originated in the convoluted course of development of Chinese medicine. The theory of the five elements was formed thousands of years ago, and its application to medicine was first recorded in The Medical Classic of the Yellow Emperor, a product of the Warring States Period. The theory of vital fire was proposed by leading physicians in the 16th century. Since then, the Kidneys have been taken as the "organ of water and fire," in which "water" refers to its yin aspect, and "fire" to its yang aspect. This doctrine concerns yin, yang, water and fire within one internal organ; though related to the five elements theory, it cannot be placed on a par with the latter.

Therefore, there have been controversies concerning the validity of the five elements theory. The main objection is to the classification into five categories. For example, the ordinary divisions of the year according to the weather in the temperate zone are the four seasons (spring, summer, autumn and winter) and in the tropical zone only two seasons (dry and rainy). The reason for adding late summer in order to make up the number of seasons to five is far-fetched. However, the quintessence of this theory is not that the number of five should be held sacrosanct, but that the mutual regulatory relationship between different types of matter should be recognized. The inter-promoting (producing) and inter-acting (checking) relationships among different systems or organs, to a great extent, are comparable with the theory of feedback control in Western medicine, but their application to the explanation of physiological phenomena and pathological changes has a much broader spectrum than the feedback mechanism. They can be taken as the basic rules of self-regulation of the human body at various levels. In fact, in modern physiology the significance of feedback control is no longer limited to research into neuro-humoral regulation, it has also been applied to research into immunology, molecular biology, genetic information and gene regulation. In a word, the five elements theory can be regarded as a general rule of correlation and coordination among the various parts of the human body and their functions, though the detailed description may not be precise.

(Source: Practical Traditional Chinese Medicine, Foreign Languages Press, 2000)