In the ancient country of Chu there was a weapons merchant with a small shop in the market. One day he went into the street to sell some spears and shields. He held up a shield and said to the crowd gathered around him, “This is the toughest shield in the world. Nothing, no matter how sharp, can ever penetrate it.”
Then he held up a spear, and said: “This is the sharpest spear under heaven. No matter how tough something is, this spear can slice right through it.”
The onlookers heard this and laughed. One of them asked him, “Well then, if your spear is thrust at your shield, what happens?”
The weapons dealer had no answer, and left looking discouraged.
Meaning: Making a contradictory statement or claiming the impossible.
自相 zìxiāng = each other, mutual
矛 máo = spear
盾 dùn = shield