以人为镜

唐太宗 said it in 643, mourning his outspoken minister 魏征: 以铜为镜，可以正衣冠；以古为镜，可以知兴替；以人为镜，可以明得失 'with bronze as a mirror you can straighten your robes, with the past as a mirror you can understand rise and fall, with a person as a mirror you can see your own rights and wrongs'. With Wei Zheng gone, he said, he had lost a mirror. The idea is older: 《墨子》 already says a gentleman uses people, not water, as his mirror.

Today it means learning from how other people succeed and fail, or from their criticism, and it is usually quoted with its second half, 可以明得失. Learning from history is 以史为镜, a modern rewording of the second mirror.