The theory of yin and yang states that all things have two opposite aspects, which are both opposite and at the same time interdependent. This is a universal law of the material world. These two aspects are in opposition to each other but because one end of the spectrum cannot exist without the other they are interdependent.
The ancient Chinese used water and fire to symbolize yin and yang; anything moving, hot, bright and hyperactive is yang, and anything quiescent, cold, dim and hypoactive is yin.
The yin and yang properties of things are not absolute but relative. These two opposites are not stationary but in constant motion. If we imagine the circadian rhythm, night is yin and day is yang; as night (yin) fades it becomes day (yang), and as yang fades it becomes yin. Yin and yang are therefore changing into each other as well as balancing each other.