"Take a person as a mirror and you can know your gains and losses" means that you can know your own gains and losses by taking a person as a mirror. This sentence comes from the Biography of Wei Zhi in the Old Tang Dynasty.
Original text:
Biography of Wei Zhi in the Old Tang Dynasty
Author: Liu Shao
Dynasty: Jin
Sighed: "Take copper as a mirror, you can get dressed properly; Take the past as a mirror, you can know the rise and fall, and take people as a mirror, you can know the gains and losses. I have tried to protect these three lessons, and I have defended myself from the inside. Today, when Wei Zhi dies, he will die. "
Translation:
Emperor Taizong sighed and said, "Using copper as a mirror can make you dignified and neat when you wear it; Taking history as a mirror, we can know that since the dynasties, survival has risen and fallen; Take people as mirrors, you can know your own gains and losses. I used to keep these three mirrors to prevent my own mistakes. Now that Wei Zhi is dead, I have lost a mirror. "
Extended information:
"Take people as a mirror, you can know the gains and losses", which emphasizes more the initiative of Emperor Taizong, and more the subjective initiative of himself, rather than the passivity of people. Taking copper as a mirror and dressing properly means that people look in the mirror, not the mirror to look at me; Taking history as a mirror and knowing the rise and fall is that people learn history, rather than taking the initiative to find someone with historical knowledge.
in the same way, in the same logic, people learn from others and know the gains and losses, which means that people take the initiative to "learn from others" and learn the gains and losses. The first two points are about initiative and consciousness, and "learning from others" of course emphasizes initiative and consciousness more.
On the whole, what Emperor Taizong emphasized is more about introspection, more about "being clear-minded and understanding", more about Confucius' self-denial, more about Confucius' "seeing the sages in Si Qi, and introspecting yourself if you don't see the sages", more about Ceng Zi's "I will save myself in three provinces", and more about Mencius' "opposing myself".