In ancient China, spears and shields were common weapons: the spear, a long pole with a sharp blade for attacking, and the shield, made of tough material for defense, both sold in markets.
One day, a man carried some spears and shields to the market to sell them. He spread these weapons on the ground for people to examine and choose. After some time, there were only onlookers, but no buyers.
The seller, afraid people wouldn't recognize its quality, picked up a shield and shouted, "Everyone, take note! This shield is incredibly sturdy—no sharp weapon can pierce it. Buy now!"
Most onlookers were skeptical, some even reaching out to touch the shield, yet still no one stepped forward to buy.
The seller, seeing that no one wanted to buy his shield, picked up a spear from the ground, brandished it, and shouted loudly, "Please take another look at this spear—how strong and sharp it is! It can pierce through anything. Buy it now!"
Some praised it, some doubted it, but most just snickered. One man picked up a shield and a spear, pressing the tip against the shield, and asked the merchant, "Hey, are you serious about what you're saying?" "Of course I am!" the merchant replied confidently. The man then demanded, "So if I take the spear you boast about and stab it into the shield you boast about, what happens then?"
The merchant, utterly embarrassed and left speechless, quickly packed up his spear and shield and hurried away.
This story vividly depicts someone who only boasts about themselves, contradicting their own words and ending up outsmarting themselves. Later, people summarized it as "self-contradiction," a common idiom in speech and writing.
Source: *Han Feizi*, Chapter "Nan Yi"
Meaning of the Idiom: Later, the Chinese idiom "自相矛盾" came to describe how someone only boasts about themselves, contradicting their own words and ending up outsmarting themselves.