What does Tang Taizong's famous saying "Learn from others, and you can know the gains and losses" mean?

It means: only by taking the success or failure of others as your own reference can you know what you should do.

Language: "Biography of Wei Zhi in the Old Tang Dynasty", the whole sentence is "A husband can wear a bronze mirror; Taking history as a mirror, we can know the rise and fall; Taking people as a mirror, you can understand the gains and losses. "

This sentence was said by Emperor Taizong after Wei Zhi's death.

Wei Zhi is a famous saint in history. On one occasion, Wei Zhi protested strongly. Emperor Taizong was very angry and wanted to kill Wei Zhi. Seeing that he was so angry that only Wei Zhi could let him do this, the eldest grandson immediately put on very dignified and formal clothes, then went to the front of Emperor Taizong and knelt down and said to him, "Congratulations to the emperor! Congratulations to the emperor! " Emperor Taizong was very angry and puzzled by the queen's actions. Then the queen said, "Your Majesty, there must be a wise Lord. Only when a wise monarch and a wise Lord appear can the courtiers dare to speak so bluntly.". As a result, Emperor Taizong turned from anger to joy: Then I am the master of wise SHEN WOO. In this way, Wei Zhi's life was saved.

Later, when Wei Zhi died, Emperor Taizong cried very sadly. He said: I have three mirrors, "I can dress with copper as a mirror", and I can put on his clothes and his hat; "Taking history as a mirror", observing his own administration with history, "knowing the rise and fall", how to make the dynasty prosperous and how to make the dynasty perish, he can observe from history; "Taking the people as a mirror" requires a wise minister to convince him to "know the gains and losses", so that you can understand the gains and losses of all his policies and then correct them. Emperor Taizong said that he had these three mirrors. When Wei Zhi died, one side was broken.