The theory of yin-yang originated in antiquity in China. It is a theory dealing with the origination of the universe as well as the motion and variation of all things in the natural world. It holds that the natural world is made up of materials and that the material world conceives, develops and constantly varies under the interaction of yin and yang. The philosophers and doctors in ancient China explained all the phenomena and the nature of the universe and life with the theory of yin-yang. They regarded the opposition, the wane and was as well as the variation of yin-yang as "the law of the universe".

1. The implication of yin-yang & the categorization of things according to the nature of yin-yang

People in ancient China held that the original state of the universe was “Qi” and that the motion and variation of “qi” produced two poles knows as “yin” and “yang”, such a process of transformation was called “to divide one into two”. Since all the things in the universe are produced through the motion and variation of qi, everything can be divided into the aspects of yin and yang, such as the heaven and the earth, the day and the night, the water and fire, upper and lower, cold and heat as well as man and woman, etc.

 1.1 The implication of yin-yang

The original meaning of yin and yang is simple and specific, mainly referring to the sides facing and opposite to the sun. That is to say that the things facing the sun pertain to yang while the things opposite to the sun pertain to yin. In Chinese “yang” means “sunshine” while “yin” means “shadow”. Later on specific things related to yin and yang were abstracted to induce a series of properties in the light of yin and yang. In this way yin and yang, two special signifiers, gradually evolved into a theory of extensive application. Consequently the implication of yin and yang was extended.

The properties of things signified by yin and yang are quite abstract. In order to make the meaning of yin and yang explicit, people in ancient China used specific things, namely water and fire, as metaphors to analogize. Since water and fire are opposite to each other in nature and reflect the basic characteristics of yin and yang, they are used as the signifiers of yin and yang in Huangdi Neijing. Comparatively speaking, fire is warm, bright, active and up-flaming; while water is cold, dim, static and downward-moving. That is why it si stipulated in Huangdi Neijing that “water is yin and fire is yang”.

1.2 The categorization of things according to yin and yang

Fire and water are the evidences used to categorize things because they are the signifiers of yin and yang, Generally speaking, the things and the phenomena that bear the properties of being warm, bright, active, rising the dispersing pertain to yang; while the things and the phenomena that bear the properties of being cold, dim, static, descending and astringing pertain to yin.

According to such criteria, all things and phenomena can be categorized into either yin or yang group. However, the yin and yang properties of things are relative, not absolute. In the categorization of things according to the nature of yin and yang, two point have to be taken into consideration.

(1) the yin or yang properties of things may vary with the change of time and application. Take spring and summer for example, it is comparatively hot in summer and cold in spring, so summer pertains to yang and spring to yin. Take spring and winter for another example. It is comparatively cold in winter and warm in spring, so winter pertains to yin while spring to yang.

(2) Any aspect of yin and yang in an object can be further and infinitely divided. In this case there exists yin within yang and yang within in. take daytime and night for example, daytime can be further divided into two phases: morning and afternoon. Since yang-qi ascends in the morning and descend in the afternoon, morning pertains to yang (yang within yang) and afternoon to yin (yin within yang). Similarly, night can be divided into anterior night and posterior night. Since yin-qi increases in the anterior night and decreases in the posterior night, anterior night pertains to yin (yin within yin) while posterior night to yang (yang within yin).