OPPOSITION AND BALANCE

The yang transforms and the yin conserves. The yang and the yin manifest as movement and rest: yang moves to its utmost, then rests; yin rests to its utmost, then moves. Therefore, yin rests within yang, and yang moves within yin; the two are inseperably interwoven. It is thus a single unit that they are one with the Dao. - Chu Xi

According to Chinese philosophy there are two basic principles, and while they sometimes oppose, they are nevertheless dependent upon each other. The two principles are yin and yang. Although yin and yang may at times be opposites, they are not hostile to each other, each has need of the other, for without one, the other could not exist, nor is one superior to the other.
The terms "yin" and "yang" first appeared in Yi Jing, The Book of Change, sometime around 1250 BC. The character 'yin' literally means "the shady side of the hill" and 'yang' means "the sunny side of the hill". These pictographs not only represent a form of opposites in polarity, but they also express transformations from one to the other, such as in seasons.
The ceaseless interplay of heaven and earth gives form to all things. The sexual union of male and female gives life to all things. This interaction of yin and yang is called Dao, the resulting creative process is called change.