Zhuangzi and Keiko play together on Haoshui Bridge. Zhuangzi said: "How leisurely the silver carp swims in the river is the happiness of the fish." Keiko said, "You are not a fish. How do you know that fish is happy? " Zhuangzi said, "You are not me. How do you know that I don't know that fish are happy? " Keiko said, "I'm not you, but I don't know you." You are not a fish, then you certainly don't know the happiness of fish. "Zhuangzi said," let's go back to the original topic. If you start asking me' where do you know the happiness of fish', it means that you know I know, so you ask me where I know. Now I tell you, I know it on Haoshui Bridge. "