Though the earthworm has neither the advantage of claws and teeth nor the strength of muscles and bones, it can eat dust and dirt above ground and drink from the waters of the Yellow Springs below, because its mind is fixed on a constant end. The crab has eight legs and two claws; still if there is no hole made by an eel or snake, it will have no safe place to live, because its mind moves in every direction at once.

For these reasons, if there is no dark obscurity in purpose*, there will be no reputation for brilliance; if there is no hidden secretiveness in the performance of duties, there will be no awe-inspiring majesty in achievements. If you attempt to travel both forks of a road, you will arrive nowhere, and if you attempt to serve two masters, you will please neither.

-- Xunzi, An Exhortation to Learning (勸學) 1.6, translated by John Knoblock in "Xunzi: A Translation and study of the Complete Works"

*Knoblock gives "If there is no ardor and enthusiasm in purpose" as an alternative, personally I would translate it as "if there is no one who deeply wills it" and similarly the next passage as "if there is no one who singlemindedly labors for it" (Knoblock doesn't give any alternative there).