During the Spring and Autumn Period, there was a man in the State of Qin whose name was Sun Yang. Sun Yang was very expert in looking at horses and judging their worth. Whatever the horse might be, he could tell whether it was good or bad at first sight. People called him Bo Le (Bo Le was the name of one of the celestial bodies and was fabled to be in charge of heavenly steeds), and he was often asked to appraise and select horses.
One day, when Sun Yang was passing a place, an old horse pulling a cart loaded with salt suddenly neighed to him without stopping. He came near, and saw that it was a horse that really could cover a thousand Li a day, and that the only problem with it was that it was a little too old. The old horse was pulling the heavy cart with difficulties and hardships. Sun Yang felt acutely that the horse was really unjustly treated, for it might have been a fine steed galloping on the battlefield. It was a great pity that it was pulling the cart loaded with salt without attracting public attention, which had taken the edge off its spirit and consumed its energy. When he thought of this, he was so grieved that he shed tears.
In order to help more people learn how to appraise horses so that fine horses which could cover a thousand LI a day would no longer fall into oblivion, and also in order to ensure that his unique skill in judging horses would not be lost, Sun Yang worte a book entitled The Art of Looking at Horses and Judging Their Worth, based on his experiences and knowledge accumulated over the years. The book was also illustrated with the pictures of various horses.
Sun Yang had a son who, after reading his father's The Art of Looking at Horses and Judging Their Worth, thought it was very esay to appraise horses. So he took the book with him to look for fine horses everywhere. At first he searched according to the pictures in the book, and accomplished nothing. Then he searched according to the characteristics of a toad fit very well the characteristics described in the book. So he happily took the toad back home, and said to his father, "Father, I have found a horse that can cover a thousand Li a day, only its hoofs are not good enough." Looking at the toad, Sun Yang did not know whether he should laugh or cry. Knowing that his son was stupid, Sun Yang said humourously. "It's a pity that this horse is too fond of jumping to pull a cart." Then he sighed, "That is just what we call looking for a steed with the aid of its picture."
Later, people have used the set phrase "look for a steed with the aid of its picture" to refer to handling affairs mechanically in the outmoded ways without being flexible. Somethimes it is also used to refer to trying to locate something by following up a clue. This set phrase originates in Lumbering in the Forest of Art written by Yang Shen in the Ming Dynasty （1368-1644）.