If a fish is not a fish, it means you are not a fish. How do you know a fish is happy? The deeper meaning is that people can't know the happiness of fish, things are unknowable, and finally come to the conclusion of agnosticism. Since things are unknowable, there is no need for people to ask for trouble and spend their energy on the pursuit of knowledge. If you must pursue infinite knowledge, you will inevitably fall into endless troubles. There are many well-known famous sentences in Zhuangzi, which can be said to be well-known, such as: "My life is limited, but my knowledge is limited", "Love is better than forgetting the Jianghu", "If life between heaven and earth is fleeting, it just happens suddenly", "Smile at each other and never go against my heart", "Heaven and earth coexist with me, and everything is one with me".
Zifei fish knows the happiness of fish, which means, "You are not a fish, how do you know whether a fish is happy?" It's easy to understand.