Without physical substance, the nature (of man and things) cannot be complete. Without nature, physical substance cannot be produced. The yang has the yin as its physical substance and the yin has the yang as its nature. Nature is active but physical substance is tranquil. In heaven, yang is active while yin is tranquil, whereas in earth yang is tranquil while yin is active. When nature is given physical substance, it becomes tranquil. As physical substance follows nature, it becomes active. Hence yang is at ease with itself but yin is fast moving without control.

(Shao Yong)