Yin and yang is the basic law of the universe.

These two fundamental forces of the universe exist in all things ever opposing and supplementing.

Interdependent on each other, relatively waxing and waning, also inter transforming into each other.

These opposite pairs show that yin and yang exist within all the phenomena’s. Yin and yang not only oppose but also contain each other, without the other, neither can exits.

Functional movements are yang and the Nourishing substance are yin, if the intestines and other internal organs do not move, "nourishing substance" cannot be digested or absorbed; and if over a long period "nourishing substances" are not provided, the organs cease to move.

The book, Canon of Medicine says: "Yin in the interior is the guardian of yang, and yang in the exterior is the activator of yin". This also shows the relationship of interdependence between yin and yang.

Yin and yang opposing each other and yet depending on each other for existence, they are not stagnant but in a dynamic state, i.e., while yin wanes, yang waxes, and vice versa. This dynamic change of succeeding each other between yin and yang is known as the waxing and waning of yin and yang.

By "transformation" it means yin and yang will transform into each other under certain conditions. For instance, in the course of suffering from a disease, the patient runs a high fever, has a red complexion, feels irritable and restless, But all of a sudden, he feels listless, his temperature becomes lower, his face turns pale. This is an example of transformation between yin and yang.

It should be pointed out that the yin or yang property of things is not absolute but relative. This relativity of yin and yang is shown in the inter transformation between yin and yang as mentioned above.

It is also shown in the constant divisibility of yin and yang, i.e., either yin or yang can be still divided into another pair of yin and yang. For instance, day is of yang nature and night is of yin nature, but both day and night can be further divided like: the period from dawn till noon is the yang aspect of yang; and the period from noon till dusk is the yin aspect of yang; the period from dusk till midnight is the yin aspect of yin; the period from midnight till dawn is the yang aspect of yin.

It is used to explain the tissues and structures, physiology and pathology of the human body, and also to the clinical diagnosis and treatment.

The relationship between yin and yang also remains constant among the five viscera the heart and lung are yang, because of being higher, while the liver, spleen and kidneys are yin, because of being lower. When speaking of yin and yang of the channels and collaterals, the channels may be divided into channels belonging to yin and yang channels, Collaterals can also be classified as yin collaterals and yang collaterals.

Out of the twelve channels, three are three yang channels of the hand and three yin channels of hand, three yang channels of the foot and three yin channels of the foot. Vital energy (qi) is yang, blood is yin.

Normal physiological functions of the human body result from the opposite, unitive and coordinate relation between yang (function) and yin (substance). Yin and yang are always in the state of dynamic balance it is known as “Yin is even and well while yang is firm, hence a relative equilibrium is maintained and health is guaranteed”.

Physiological function is based on substance, without substance, including essence of life, blood; constructive energy and body fluid, there would be no source for functions. But the metabolism of substance must depend on the performance of function. If yin and yang of the human body separate from each other and cannot aid each other life will come to an end. This is what is called, "the divorce of yin and yang means the end of one's life."

Imbalance of yin and yang is one of the basic pathogeneses of a disease. For example, preponderance of yang leads to hyper function of the organism and heat manifestations, while preponderance of yin leads to hypo function of the organism or endogenous cold.

Deficiency of yang brings, on symptoms of external cold, while deficiency of yin as the result of exhausted vital essence leads to endogenous heat.

Yang in excess makes yin suffer, while yin in excess makes yang suffer. In severe damage of yang, yin is involved, in severe damage of yin, yang is involved too. Thus, yin and yang are both damaged.

A case of yin nature (hypo function) in its extreme may show yang (hyper function) symptoms and signs, while a case of yang nature (hyper function) in its extreme will give rise to yin (hypo function) symptoms and signs

Because one of the basic pathogeneses of a disease is the imbalance of yin and yang, any disease, no matter how intricate and volatile its clinical manifestations can be diagnosed with the theory of yin and yang.

In making a diagnosis, the first important thing is to ascertain whether the disease is yin or yang.