The phrase comes from a story. Once upon a time there was a man of a state of Chu who had a spear and a shield for sale. In praising of his shield he said “my shield is so solid that nothing can cut it through.” He also praised his spear “my spear is so strong that it can pierce through anything.” Right now, a man came and asked “what will happen if your spear is used to pierce your shield?” He was unable to give an answer and escaped from the market as soon as possible.
It is impossible for the strongest shield to coexist with a spear that finds nothing impenetrable.
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