During the Warring States period, the master craftsman Lu Ban was commissioned by the state of Chu to build a siege ladder for attacking the state of Song. When Mozi, a philosopher who championed universal love and opposed war—and who had himself worked as a carpenter, building chariots and defensive devices—heard of this, he traveled from the state of Qi for ten days and nights to meet Lu Ban in Chu.
Mozi said to Lu Ban, "Someone has bullied me—I want you to kill him for me." Lu Ban, displeased, replied, "I have never killed anyone." Mozi pressed, "Now Chu is about to invade Song, and the war will cost many lives. You are building siege weapons for Chu—if that isn't killing, what is?" After a moment, Lu Ban admitted, "But I have already built the cloud ladders." Mozi said, "Take me to see the King of Chu—I will persuade him."
Together, the two went to meet the King of Chu.
Mozi asked the King of Chu, "If a wealthy man rides in a fine carriage, wears good clothes, and eats rice and pork, yet still steals his neighbor's broken cart, coarse garments, and rice chaff, what kind of person would you say he is?"
“He’s a thief,” said the King of Chu.
"The State of Chu has a territory of 5,000 li with abundant resources, while the State of Song has only 500 li with scarce resources. For Chu to invade Song—how is that any different from that rich man playing the thief!"
"Your words make sense, but Luban has already built the cloud ladder, and I want to test its power in this war." "Very well, let Luban represent the attacking state of Chu, and I represent the defending state of Song. Let's try it right here."
Mozi then took off his belt and laid it on the ground to represent a city wall, using chopsticks as scaling ladders for the Chu army and defensive weapons for the Song army. He challenged Lu Ban to attack. Lu Ban tried nine different tactics to storm the city, but Mozi repelled every single one.
Lu Ban, his face flushed, said to Mozi, "Don't be smug. I have a way to deal with you, but I won't say it." Mozi stood his ground and replied, "I know how you plan to deal with me, but I won't say it either." King Chu, unable to understand them, asked Mozi, "What are you two talking about?"
Mozi replied, "Lu Ban wants to kill me, thinking that without me, no one would help Song defend its city. Little does he know I have already sent 300 disciples to Song, waiting for your attack!"
King of Chu, seeing no certainty of victory in attacking the Song state, said, "Enough, I have decided not to fight this battle." Later, because Mozi was skilled in defending cities, skilled defenders were called "Mozi's defense," which evolved into the idiom "Mozi's rigid defense," meaning sticking to old ways and refusing to improve.
Source: *Mozi*, Chapter "Gongshu"
Meaning of the Idiom: Later, the Chinese idiom "墨守成规" came to describe how sticking to old ways and refusing to improve.