《论衡·卷十一·说日篇》
就是有学问、有口才的人，也都把它归结为不容易弄明白的问题，不肯辨说清楚。
通人谈士，归於难知，不肯辨明。
Even those who are knowledgeable and eloquent people, all conclude that this is a problem that is not easy to understand, and they are unwilling to explain it clearly.