郑人买履

A man of the state of Zheng wanted to buy a pair of shoes. He measured his foot and put the measurement on a chair. When he set out for the market he forgot to bring it along. It was after he had found the pair he wanted that this occurred to him.

I forgot the measurement, said he.

He went home to get it but when he returned the market had broken up and he did not get his shoes after all.

Why didn't you try on the shoes with your feet?

He was asked.

I 'd rather trust the measurement than trust myself.