The story of “The Debate on the Joy of Fish” is a well known anecdote that has been compared to the Socratic dialogue tradition of ancient Greece.
Zhuangzi and Huizi were enjoying themselves on the bridge over the Hao River. Zhuangzi said, “The minnows are darting about free and easy! This is how fish are happy.”
Huizi replied, “You are not a fish. How do you know that the fish are happy?” Zhuangzi said, “You are not I. How do you know that I do not know that the fish are happy?”
Huizi said, “I am not you, to be sure, so of course I don’t know about you. But you obviously are not a fish; so the case is complete that you do not know that the fish are happy.”
Zhuangzi said, “Let’s go back to the beginning of this. You said, How do you know that the fish are happy; but in asking me this, you already knew that I know it. I know it right here above the Hao.”
— Zhuangzi, chapter 17 (Watson translation)
The exact point made by Zhuangzi in this debate is not entirely clear. The story seems to make the point that “knowing” a thing is simply a state of mind, and that it is not possible to determine if that knowing has any objective validity. This story has been cited as an example of Zhuangzi’s linguistic mastery, as he subtly uses reason to make an anti-rationalist point.