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 wax as well as the variation of yin-yang as "the law of the universe".

The implication of yin-yang and 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 known 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.