How can you know the joy of fish if you are not a fish?

Lianjiang Academy is also located in Linpu Village. The building is connected to Taishan Palace and is a well-preserved ancient academy. There is a stone beam that is said to be a thousand years old. The academy is a place for teaching and learning, so it is relatively simple and has no valuable objects. However, there is an interesting stone mortar for washing brushes with three large characters engraved on it: "Zhi Yu Le" (Know the Joy of Fish). We have all heard the saying: "How can you know the joy of fish if you are not a fish?" The more I think about it, the more interesting these three characters become. What kind of state of mind is this?