YIN AND YANG:
THE principle of Yin and Yang is that all things exist as inseparable and contradictory opposites, for example, female-male, dark-light and old-young. The principle, dating from the 3rd century BCE or even earlier, is a fundamental concept in Chinese philosophy and culture in general. The two opposites of Yin and Yang attract and complement each other and, as their symbol illustrates, each side has at its core an element of the other (represented by the small dots). Neither pole is superior to the other and, as an increase in one brings a corresponding decrease in the other, a correct balance between the two poles must be reached in order to achieve harmony.
The concept of Yin and Yang became popular with the work of the Chinese school of Yinyang which studied philosophy and cosmology in the 3rd century BCE. The principal proponent of the theory was the cosmologist Zou Yan (or Tsou Yen) who believed that life went through five phases (wuxing) – fire, water, metal, wood, earth – which continuously interchanged according to the principle of Yin and Yang.