Once in the ancient Kingdom of Zheng there was a man who wanted to buy himself a pair of shoes. He took the measurement of his own feet with a piece of string.Then he went to the market. He was looking at some shoes at the shoe-maker's when he realized he had left the measurement at home. He put down the shoes and rushed home to fetch it. By the time he returned with the measurement the market had been closed. So he failed to get his shoes.
Somebody asked him, "Why didn't you just try the shoes on your feet?" He answered, "I would rather trust the measurement than my feet."