刻舟求剑

A man of Chǔ was crossing a river in a boat. Midway across, his sword slipped from its scabbard and fell into the water. He quickly drew his knife and cut a notch into the rail of the boat, at the spot directly above where the sword had fallen. "This," he said, "is where my sword went in."

When the boat reached the far shore, the man slipped over the side at the notch he had carved, and dived down to search for his sword.

The story is told in the 吕氏春秋 (Lǚshì Chūnqiū, "The Spring and Autumn of Mr. Lǚ"), a third-century BCE encyclopedic work sponsored by Lǚ Bùwéi, chancellor of Qin and regent to the young man who would later become the First Emperor. The parable appears in the chapter 察今 ("Chá jīn," "Examining the Present"), which argues that laws and institutions must be revised as circumstances change. The sword is in one place. The boat has moved. The mark on the boat tells you nothing about where the sword is now.

The chengyu names a specific logical error: using a reference point that was valid at one moment to locate something at a later moment, after the relationship between the reference and the thing has changed. It is not merely about being wrong; it is about treating a transient indicator as though it were permanent, and so becoming systematically wrong in a way the original circumstances have already ruled out.

The idiom shares territory with 守株待兔 ("guarding the stump") but points in a different direction. 守株待兔 is the error of expecting a rare event to repeat. 刻舟求剑 is the error of treating a contingent relationship as if it were fixed. One is about probability; the other is about reference frames. Both, in the Warring States philosophical context, were arguments against applying old methods to new circumstances.