从前，有一个卖矛和盾的楚国人。他大声的夸奖他的盾："我的盾是最坚固的，没有东西能刺透它。" 接着，他又赞美他的矛， 说："我的矛是最锐利的，它能刺透任何东西。"有人问他："那如果用你的矛去刺你的盾，会怎么样呢？"
卖矛和盾的人回答不出来了。
    世界上不可能同时存在最坚固的盾和最锐利的矛。
(《韩非子》)

His spear against his shield

A man of the state of Chu had a spear and a shield for sale. He was loud in praises of his shield.
"My shield is so strong that nothing can pierce it through."
He also sang praises of his spear.
"My spear is so strong that it can pierce through anything."
What would happen, he was asked, if your spear is used to pierce your shield?
He was unable to give an answer.
It is impossible for the strongest shield to coexist with a spear that finds nothing impenetrable.
(《Hanfeizi》)