The Way of Heaven is called the Round; the Way of Earth is called the Square. The square governs the obscure; the circular governs the bright. The bright emits qi, and for this reason fire is the external brilliance of the sun. The obscure sucks in qi, and for this reason water is the internal luminosity of the moon. Emitted qi endows; retained qi transforms. Thus yang endows and yin transforms. The unbalanced qi of Heaven and Earth, becoming perturbed, causes wind; the harmonious qi of Heaven and Earth, becoming calm, causes rain. When yin and yang rub against each other, their interaction produces thunder. Aroused, they produce thunderclaps;
disordered, they produce mist. When the yang qi prevails, it scatters to make rain and dew; When the yin qi prevails, it freezes to make frost and snow. Hairy and feathered creatures make up the class of flying and walking things and are subject to yang. Creatures with scales and shells make up the class of creeping and hiding things and are subject to yin. The sun is the ruler of yang. Therefore, in spring and summer animals shed their fur; at the summer solstice, stags’ antlers drop off. The moon is the fundament of yin. Therefore when the moon wanes, the brains of fish shrink; when the moon dies, wasps and crabs shrivel up. Fire flies upward; water flows downward.
Thus, the flight of birds is aloft; the movement of fishes is downward. Things within the same class mutually move one another; root and twig mutually respond to each other. Therefore, when the burning mirror sees the sun, it ignites tinder and produces fire. When the square receptacle sees the moon, it moistens and produces water. When the tiger roars, the valley winds rush; when the dragon arises, the bright clouds accumulate. When qilins wrangle, the sun or moon is eclipsed; when the leviathan dies, comets appear. When silkworms secrete fragmented silk, the shang string [of a stringed instrument] snaps. When meteors fall, the Bohai surges upward.