《韩非子·外储说左上》
这是君主长期受欺骗，而游说的人长期被供养的原因。有个愿教燕王学习长生不死道术的客人，燕王派人去向他学习。派去学习的人还没来得及学到手，那个客人先死了。
客有教燕王为不死之道者，王使人学之，所使学者未及学而客死。
This is the reason why monarchs have long been deceived, while those who lobby have long been supported. A guest willing to teach the King of Yan the art of eternal life came, and the King of Yan sent a person to study with him. The one sent to study had not yet had the time to learn anything, but the guest died first. 