Looking For A Horse With The Aid Of Its Picture

The Chinese of Looking For A Horse With The Aid Of Its Picture--An Tu Suo Ji is 按图索骥。
Bo Le whose real name was Bo Le was the most remarkable connoisseur for horse in Chinese Spring and Autumn Period. Bo Le was his nickname which was from the name of an immortal who was fabled to be in charge of heavenly steeds.

In order to ensure that his unique skill would not be lost, and also in order to help more people learn how to appraise horses, Bo Le wrote a book entitled The Art of Appraising Horses, based on his experiences and knowledge accumulated over the years. The book was also illustrated with the pictures of various horses.
Bo Le had a son keen on being a good horse connoisseur like his father. Having read the book, he thought he had already mastered all skills of his father. So he took the book with him to look for fine horses. On the way, he found a toad and noticed its characteristics fit very well the characteristics described in the book. 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."
Disappointed at his son's silliness, Bo Le did not know if he should cry or laugh. So he said ironically. "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 horse with the aid of its picture."

Background and Writer Comment:

Now this idiom is used 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.