《淮南子·道应训》
所以《慎子》说： 工匠知道做门，但如果不知道怎样使门能开能关，这就等于不知道做门的关键所在。所以一定要知道这门之关键所在，才能算会做门。 
故慎子曰： 匠人知为门能以门，所以不知门也，故必杜然后能门。 
Therefore, Shen Buhu said: A craftsman knows how to make a door. However, if he doesn’t know how to make the door open and close, it means that he doesn’t know the key point of making a door. Therefore, knowing the key point of making a door is an essential part of making a door.