This sentence was said by Li Shimin, Emperor Taizong of the Tang Dynasty. It comes from "Old Tang Book Wei Zheng Biography". The original sentence is:

Taizong said to Liang Gong: "Use copper as a mirror. You can correct your clothes; use the past as a mirror to know the rise and fall; use people as a mirror to know the gains and losses. I have tried these three mirrors to guard against my mistakes. Now the Wei Dynasty has passed away, and then one mirror has died. "

Taizong said to Liang Gong: "Use copper as a mirror, and you can wear your clothes and hats properly; use history as a mirror, and you can know the reasons for the rise and fall of a country; use people as a mirror, You can discover your own rights and wrongs. I regard these three kinds of mirrors as treasures to prevent myself from making mistakes. Now that Wei Zheng has passed away, I am missing a good mirror."

Extended information:

Wei Zheng is famous for his outspokenness and daring to remonstrate. According to statistics recorded in "Zhenguan Politicians", Wei Zheng presented remonstrances to Li Shimin fifty times and submitted eleven memorials to Li Shimin. He gave as many as "dozens of remonstrances" throughout his life. More than ten thousand words." The number of followers, the passionate words, and the firm attitude are all unmatched by other ministers.

Wei Zheng (the second year of Xiangxiang in the Northern Zhou Dynasty (580) - the seventeenth year of Zhenguan in the Tang Dynasty (February 11, 643)), named Xuancheng, was born in Quyang County, Julu County (now Jinzhou City) ). Politicians, thinkers, writers and historians of the Sui and Tang Dynasties were called "famous prime ministers of a generation" by later generations for their outspoken advice and assisting Emperor Taizong of the Tang Dynasty in establishing the "Government of Zhenguan".

When he reached the official position of Guanglu Doctor, he was granted the title of Duke of Zheng, with the posthumous title of "Wenzhen". The funeral was simple. In February of the same year, Li Shimin ordered Yan Liben to paint twenty-four portraits of heroes and place them in Lingyan Pavilion, with Wei Zheng ranking third.

He is the author of the preface to Sui Shu and the general introduction to Liang Shu, Chen Shu and Qi Shu. Most of his remarks can be found in "Zhenguan Zhengyi". Among them, the most famous and handed down admonishment list is "Ten Thoughts on Admonishing Taizong".

Reference materials: Baidu Encyclopedia - Using copper as a mirror, Baidu Encyclopedia - Using people as mirrors can reveal gains and losses