The theory of yin and yang is an ancient philosophical theory that studies the connotation of yin and yang and the laws of its movement and change. Yin and yang are used to explain the occurrence, development, and change of everything in the universe.

The theory of yin and yang holds that the world is a material whole, and the world itself is the result of the union of opposites of yin and yang. The interaction of yin and yang contributes to the occurrence of all things and promotes the development and change of everything.

All things or phenomena in the universe that are interrelated and opposed to each other, or two aspects of the same thing that are opposed to each other are applicable to yin and yang which summarize and analyze their respective properties.

Generally speaking, anything that is moving, outward, ascending, warm, intangible, bright, and exciting belongs to Yang. Relatively static, inward, descending, cold, tangible, dark, and suppressing belongs to Yin.

Common examples of Yang: Sky, father, male, light, heat, cohesion, expansion, strength.

Common examples of Yin: Water, mother, female, dark, cold, restrained, effusive, weak.

The human body is also divided into yin and yang. Different parts, tissues, and even different physiological activities of the human body can be divided into the two categories of yin and yang—for example:

According to “Somun․Eumyang-ihamnon” (a study on medical thought), yin and yang can be subdivided infinitely, and this concept is also applied in traditional Chinese medicine to explain the different states of physiological activities. For example, the five viscera are yin; the state of activity of each organ can be subdivided into yin and yang, such as liver yin and liver yang; kidney yin and kidney yang, etc.

The basic of the theory of yin and yang include yin and yang opposition and restriction, mutual use of yin and yang, sympathy and mutual storage of yin and yang, growth and decline of yin and yang, the transformation of yin and yang, and self-harmony and balance of yin and yang.

As a unique thinking method of traditional Chinese medicine, the theory of yin and yang is widely used to explain the life activities of the human body, the causes of diseases and pathological changes, and to guide the diagnosis and prevention of diseases. It has become an important part of the theoretical system of traditional Chinese medicine.