Search for a good horse according to a picture; Try to locate something by following a clue; Follow a set pattern.

During the Spring and Autumn Period (770-476BC), there was a man named Sun Yang in the State of Qin. Sun was an expert in assessing horses. Whatever the horse, Sun could tell whether it was good or bad in quality at first sight. People called him Bo Le (a celestial body in charge of the heavenly steeds), and he was often asked to appraise and select horses.

One day Sun noticed an old horse pulling a cart loaded with salt that suddenly began neighing. Sun approached the horse and noticed that although it was old it could plow 1,000 li (1 li = 0.5 km) a day. As the old horse pulled the heavy cart with difficulty, Sun felt it was being unjustly treated and that it could have made a fine steed on the battlefield. It was a great pity to Sun that the horse's hardships, which stunted his spirit and consumed his energy, did not attract any attention. Thinking about this, Sun became so grief-stricken that he began to cry.

To help people learn how to appraise horses so that fine animals like this one would no longer fall into oblivion, Sun Yang wrote an illustrated book entitled The Art of Looking at Horses and Judging Their Worth . The work, which was based on Sun's experiences and the knowledge he accumulated over the years would also ensure the art of horse appraising would never be lost.

After reading the book, Sun's son set out to look for fine horses, thinking it would be very easy to appraise them. At first he searched for horses according to the pictures in the book. Accomplishing nothing he then searched according to the animal characteristics listed in the book. Finding that the characteristics of a toad fit very well with the characteristics described in the book, he happily took the toad back home and said: "Father, I have found a horse that can cover 1,000 li a day, but its hoofs are not good enough." Looking at the toad, Sun did not know whether to laugh or cry. Knowing that his son was stupid, Sun said teasingly: "It's a pity that this horse is too fond of jumping to pull a cart. ... That is just what we call looking for a steed with the aid of its picture," he added with a sigh.

Later, people used the expression to refer to handling affairs mechanically in an outmoded way without flexibility. Sometimes the saying is also in reference to locating something by following up on a clue. The idiom originated in Lumbering in the Forest of Art by Yang Shen of the Ming Dynasty (1368-1644).