《淮南子·道应训》
所以《老子》说： 虽然知道什么是刚强，但却谨守柔弱。甘心处于天下的低卑处。 
故老子曰： 知其雄，守其雌，其为天下谿。 
Therefore, the 《Laozi》 says: Although knowing what is firm and strong, yet firmly guarding what is weak and yielding. Willingly being in the lowliest place under heaven. 