按图索骥
春秋时候，秦国有个叫孙阳的人，擅长相马，无论什么样的马，他一眼就能分出优劣。他常常被人请去识马、选马，人们都称他为伯乐（“伯乐”本是天上的星名，据说负责管理天马）。
有一次，孙阳路过一个地方，忽见一匹拖着盐车的老马冲他叫个不停，走近一看，原来是匹千里马，只是年龄稍大了点。老马拉着车艰难地走着，孙阳觉得太委屈了这匹千里马，它本是可以奔跑于疆场，可以发挥更大作用的宝马良驹，现在却默默无闻地拖着盐车，慢慢地消耗着它的锐气和体力，实在可惜！孙阳想到这里，难过得落下泪来。
为了让更多的人学会相马，使千里马不再被埋没，也为了自己一身绝技不至于失传，孙阳把自己多年积累的相马经验和知识写成了一本书，配上各种马的形态图，书名叫《相马经》。
孙阳有个儿子，看了父亲写的《相马经》，以为相马很容易，就拿着这本书到处找好马。他按照书上所绘的图形去找，一无所获。又按书中所写的特征去找，最后发现有一只癞蛤蟆很像书中写的千里马的特征，便高兴地把癞蛤蟆带回家，对父亲说：“爸爸，我找到一匹千里马，只是蹄子稍差些。”父亲一看，哭笑不得，没想到儿子竟如此愚笨，便幽默地说：“可惜这马太喜欢跳了，不能用来拉车。”接着感叹道：“所谓按图索骥也。”
故事出自明朝杨慎的《艺林伐山》。成语“按图索骥”，比喻机械地照老办法办事，不知变通；也比喻按照某种线索去寻找事物。
looking for a steed with the aid of its picture
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 acart 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）.