From Emperor Taizong of Tang's famous remark in
《旧唐书·魏徵传》 on the death of his minister Wei Zheng:
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. The triple 'three mirrors' passage: bronze mirror for appearance, history for dynastic rise and fall, and other people for one's own merits and faults.