《黄帝内经·素问·疏五过论》
道之远大幽深，好象视探深渊，又好象迎看浮云，但渊虽深，尚可以测量，迎看浮云，却不到其边际。
闵闵乎若视深渊，若迎浮云，视深渊尚可测，迎浮云莫知其际。
The way is far-reaching and profound, just like looking into a deep abyss or watching the floating clouds, but although the abyss is deep, it can be measured, while watching the floating clouds, one will never reach the end. 