《文心雕龙·书记》
天地之间阴阳五行的消长盛衰，虽然变化无常，但考察其变化是有一定法则的。所谓 律 ，就是中。
阴阳盈虚，五行消息，变虽不常，而稽之有则也。律者，中也。
<translation>The growth and decline of Yin and Yang and the five elements between heaven and earth, although their changes are impermanent, there are certain laws in examining their changes. The so-called lv is in the middle.