It means: Use the successes and failures of others as your own reference, so that you can know what to do yourself.

The quote comes from: "Old Book of Tang·Biography of Wei Zheng", the full sentence is "If you use copper as a mirror, you can straighten your clothes; if you use ancient times as a mirror, you can know the ups and downs; if you use people as a mirror, you can know gains and losses." ."

This sentence was said by Emperor Taizong of Tang Dynasty after Wei Zheng's death.

Wei Zheng is a famous wise minister in history. Once, Wei Zheng's remonstrance was so fierce that Tang Taizong was so angry that he wanted to kill Wei Zheng. Queen Changsun saw that he was so angry that he was half dead. She knew that only Wei Zheng could do this to him, so she immediately went to change into a very dignified and formal dress. Then he walked to Tang Taizong, knelt down and said to him: "Congratulations to the Emperor! Congratulations to the Emperor!" Tang Taizong was so angry that he felt baffled by the Queen's behavior. Then the Queen said: Your Majesty, a Ming Lord must appear. Only after the Ming Lord appears can the ministers dare to speak out like this. As a result, Emperor Taizong of the Tang Dynasty turned from anger to joy: Then I am the Lord of Ming Dynasty. In this way, Wei Zheng's life was saved.

Later, when Wei Zheng passed away, Emperor Taizong of the Tang Dynasty cried very sadly. He said: I have three mirrors. "History is a mirror", using history to observe one's own governance, "you can know the ups and downs", what can be done to make a dynasty prosperous, and what should be done to make a dynasty fail, he can learn from the observation of history; "taking people as a mirror", using When a virtuous minister comes to advise him, he can "understand the pros and cons" of all his policies, and then make corrections. Emperor Taizong of the Tang Dynasty said that he had these three mirrors. When Wei Zheng died, one of them was broken.