These are the five basic substances that constitute the material world.

These substances are not only of the relations with generation and restriction but set in a state of constant motion and change. That’s why it is also known as the doctrine of five evolutive phases.

A comprehensive comparison and study of all kinds of things and phenomena in nature as well as in the viscera, bowels, tissues, physiology and pathology of the human body, attributed them respectively to one of the five elements, i.e., wood, fire, earth, metal and water in accordance with their different properties, functions and forms, thus expounding the physiology, pathology of the human body and the correlation between man and his natural surroundings.

As far as the relationship of generation of each of the five elements is concerned, it is composed of two aspects generating and being generated.

The element that generates is called the mother, while the element that is generated is called the son. Hence, the relation of generating and being generated among the five elements is also known as that of mother and son.

Take the example of wood. Because wood produces fire, it is called the mother of fire. On the other hand it is produced by water, so it is called the son of water.

Each of the five elements has the chance of restricting and being restricted.

Take wood for example. The element restricting wood is metal, and the element that is restricted by wood is earth.

In view of the correlations among things, neither generation nor restriction is dispensable. Without generation there would be no birth and development; without restriction, excessive growth would result in harm. For instance, on one hand, wood generates fire, and on the other, it restricts earth; while earth, in its turn, generates metal and restricts water. Thus in generation there resides restriction, and in restriction there exists generation. They oppose each other and at the same time cooperate with each other, thus a relative balance is maintained between generation and restriction, and the normal growth and development of things are ensured.

Should one of the five elements be excessive or insufficient, there would appear the phenomena of abnormal restrictions, known as subjugation and reverse restriction.

By subjugation is meant that one element subdues the other when the latter is weak. It is the manifestation of abnormal coordination among things.

For instance, if wood is in excess and metal cannot exercise normal restriction on it, then the excessive wood will subjugate earth in such, a way that earth will become weaker.

Reverse restriction means preying upon others. That is, when any one of the five elements is in excess, the one originally restricting it will be restricted by it instead. That is why we call it reverse restriction. For instance, the normal order of restriction is that metal restricts wood; but if wood is in excess or metal is insufficient, wood will restrict metal in the reverse direction. It is clear that the order of reverse restriction is just opposite to that of restriction and that reverse restriction is undoubtedly a harmful one.

The restriction of the five elements can be used to explain the inter restraining relations between the five viscera. For instance, the lung (metal) disperses and descends so as to restrain the exuberance of the liver (wood), which is known as metal restricting wood. The liver (wood), that functions well smoothes and regulates the stagnation of the qi of the spleen (earth), which is called as wood restricting earth. The function of the spleen (earth) plays role in transporting, distributing and transforming nutrients and promoting water metabolism and may prevent the overflow of the water of the kidney (water), which is referred to as earth restricting water. The ascending of the kidney (water) can prevent the heart (fire) from hyper acting fire.

Take lung trouble for example. If it results from heart trouble, that is regarded as "fire subjugating metal”. If it is caused by liver (wood) trouble, that is refer- red to as "wood reversely restricting metal". If it is led by spleen (earth) trouble, that is looked upon as “illness of mother organ involving son organ". And if it is due to kidney (water) trouble, that is taken as “illness of son organ involving mother- organ".

As well as the relevant pulse conditions, one, when diagnosing, should do like this: first, synthesize the materials obtained through the four methods of diagnosis (observation, listening and smelling, inquiring, pulse feeling and palpation), and then, deduce what the disease is according to the theory of five elements. For instance a blue face, preference for food with sour flavor, and bowstring pulse condition may indicate liver disease; a red face, bitter taste in mouth and overflowing pulse condition may suggest the hyperactivity of heart fire; the patient whose qi of the spleen is deficient looks blue because of “wood subjugating earth”; the patient has heart trouble looks dark because of “water restricted fire” to mention a few.

In treatment, the theory of five elements, on one hand, is used to prevent the transmission of diseases. The Classic on Medical Problems says, "When liver disease occurs, it will spread to the spleen, so the qi of the spleen should be reinforced before it is affected." On the other hand, the theory of five elements may be used to help work out the principle and method of treatment. For instance, The Classic on Medical Problems, points out that "If hypo function is found in the son organ, the mother organ should be tonified; and if hyper function is found in the mother organ, the son organ should be treated with purgation”, to mention but a few.

The law of generation, restriction, subjugation and reverse restriction can be applied to work out many effective and specific treating methods such as reinforcing earth to generate metal (strengthening the function of the spleen to benefit the lung), replenishing water to nourish wood (nourishing the essence of the kidney to benefit the liver), supporting earth to restrict wood (supplementing the function of the spleen to treat hyperactivity of the liver), and strengthening water to control fire (replenishing the essence of the kidney to treat hyperactivity of the heart), and so on.

Above points are respectively included in the treatment based on five elements. In combination with the specific state of certain illness in accordance with other theories.