《吕氏春秋·纪·季春纪》
精气一上一下，环绕往复，循环不已，无所留止，所以说天道圆。
精气一上一下，圜周复杂，无所稽留，故曰天道圜。
The essence and qi go up and down, encircle and return, circulate without end, and never stop, which is why it is said that the Way of Heaven is round.