https://zhidao.baidu.com/question/1864582333185138387.html says "以铜为镜，可以正衣冠，以史为镜，可以知兴替，以人为镜，可以明得失" It means: using copper as a mirror, one can arrange one's clothes; using history as a mirror, one can know the ups and downs of history; using others as one's own mirror, one can know one's gains and losses every day The literal translation of the idiom seems to work well in English. If you want the meaning in non-idiomatic English, you may have to work harder.