The lesson is right in front of us; if later generations mourn without learning, tragedy will come again.
Tips
history
From Du Mu's (杜牧, Tang) 《阿房宫赋》: 秦人不暇自哀，而后人哀之；后人哀之而不鉴之，亦使后人而复哀后人也。(The Qin had no time to grieve for themselves, so later generations grieved for them; if later generations grieve without taking warning, they only give still later generations cause to grieve for them too.) A critique of the extravagance that doomed the Qin dynasty.
usage
鉴 here means 'to take as a mirror / warning' (借鉴). Often cited when commentators argue that historical lessons are being ignored.
