During the Spring and Autumn period, there was a man in the state of Qin named Sun Yang. He was an expert in judging horses, able to tell their quality at a glance. People called him Bo Le (a celestial figure fabled to be in charge of heavenly steeds) and often sought his appraisal.
One day, Sun Yang encountered an old horse pulling a heavy salt cart. He recognized it as a steed capable of covering a thousand li a day, now worn down by its labor. Moved by its unjust treatment, he shed tears, imagining it galloping on a battlefield instead.
To preserve his knowledge and help others identify fine horses, Sun Yang wrote a book titled The Art of Looking at Horses and Judging Their Worth, illustrated with pictures of various horses.
His son, after reading the book, thought appraising horses was easy. He took the book to search for a fine steed. Finding no match by the pictures alone, he then looked for matching characteristics. He found a toad that seemed to fit the book's description and brought it home, declaring, "Father, I've found a horse that can cover a thousand li a day, only its hoofs are not good enough."
Sun Yang, not knowing whether to laugh or cry, said humorously, "It's a pity this horse is too fond of jumping to pull a cart." He then sighed, "That is just what we call 'looking for a steed with the aid of its picture.'"
Later, this set phrase came to mean handling affairs mechanically by the book, without flexibility. It can also refer to trying to locate something by following a clue.