Looking for a Steed with the Aid Of Its Picture
During the Spring and Autumn Period (chūn qiū 春秋时期,770-476 BC), there was a man in the State of Qin whose name was Sun Yang (sūn yáng 孙阳). Sun Yang was very expert in looking at horses and judging their worth. 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.
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 thought it was very esay to appraise horses according to the book. So he took it with him to look for fine horses. He found the characteristics of a toad fit very well the characteristics described in the book. So he happily took the toad back home, Sun Yang did not know whether he should laugh or cry. "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. Sometimes it is also used to refer to trying to locate something by following up a clue.