just came across:
明鏡所以照形，往事所以知今
Míngjìng suǒ yǐ zhào xíng; gǔ shì suǒ yǐ zhī jīn.
Just as a bright mirror can be used to reflect images, past events
can be used to understand (or know) the present.
The locus classicus of this saying is the Records of Later Han (後
漢書) compiled by Fan Ye (范曄, 398-445) in the 5th century AD.
Here's the gloss given in the 國語辭典 dictionary:
(諺語)比喻鑑往足以知今。後漢書˙卷十七˙馮異傳：
愚聞明鏡所以照形，往事所以知今。昔微子去殷而入
周，項伯畔楚而歸漢……彼皆畏天知今，睹存亡之
符，見廢興之事，故能成功於一時，垂業於萬世也。
