5.自相矛盾

Spear and Shield Contradicting Each Other

    There was a man who was a seller of  spears and shields.  To advertise the firmness of his shields, he cried, "Look, nothing can penetrate my shields."  A moment later, to advertise the sharpness of his spears, he cried, "Look, there's nothing they will not penetrate." 

    "How about using your own spear to try on your own shield?" someone asked. The man could not make a reply.

Hanfeizi (Writings of Han Fei)

 

    人有鬻矛與盾者,譽其盾之堅:「物莫能陷也。」俄而又譽其矛曰:「吾矛之利,物無不陷也。」

    人應之曰:「以子之矛,陷子之盾,何如?」其人弗能應也。

《韓非子•難勢》