Qi (气), or vital force, consists of two opposing aspects. According to the Song- dynasty philosopher Zhang Zai（1020-1077）, everything in the world consists of qi. On the one hand, qi is a whole and one thing；on the other, it consists of pairs of contradictory states, such as the real and the unreal, motion and stillness, concentration and diffusion, and clarity and opacity. Without interaction between the opposite states, the whole cannot exist. Likewise, without the whole, there can be no interaction between the opposite states. Contradictions within the whole constitute the source of changes of qi and all things made of qi.

One thing with two states, that is qi, or vital force. As one whole thing, qi has miraculous movements, caused by interaction between the two opposites; and such movements create endless changes. This is why heaven has three aspects (the whole and the two states). (Zhang Zai: Enlightenment Through Confucian Teachings)