《论衡·卷十二·谢短篇》
哪么儒生不知道汉朝的事情，就是世上愚昧无知的人。复习旧的了解新的，才可以做老师。
然则儒生不能知汉事，世之愚蔽人也。 温故知新，可以为师。 
So the Confucian scholars do not know the affairs of the Han Dynasty, and they are the ignorant people in the world. Review the old and learn the new, and then you can be a teacher.