《淮南子·汜论训》
与其诵读先王的诗书，不如听他们的言论；与其听他们的言论，不如了解他们说这些言论的根据理由；而这些言论的根据和理由，又是难以用言语表达的。所以是 道可道，非常道 。
诵先王之收《诗》《书》，不若闻得其言。闻得其言，不若得其所以言。得其所以言者，言弗能言也。故道可道者，非常道也。
Instead of reading the words of earlier kings, it would be better to listen to their conversations; instead of listening to their conversations, it would be better to understand the reasons for their words; but these reasons are difficult to express in words. So, “that which can be spoken is not the eternal Way.”