近塞上之人有善术者。马无故亡而入胡。人皆吊之，其父曰：“此何遽不为福乎？”居数月，其马将胡骏马而归。人皆贺之，其父曰：“此何遽不能为祸乎？”家富良马，其子好骑，堕而折其髀（读音bì，股，大腿）。人皆吊之，其父曰：“此何遽不为福乎？”居一年，胡人大入塞，丁壮者引弦而战，近塞之人，死者十九，此独以跛之故，父子相保。故福之为祸，祸之为福，化不可极，深不可测也。——《淮南子·人间训》

An old man lost horse

There was a man with talent of philosophy, who lost his horse in a day. All people felt sad about it, he said"How couldn't this be a good thing?". Then months later, his horse lead many many wild horses back, and people all felt he was very luck. But the man said"How couldn't it turn into a bad thing?".

With so many horses to ride and play, his son fell down and can never walk again. So sadly and peole all felt sorry about him. Again the old man said"How could't this be a good thing?"

Years later, people all were involved in the war. Healthy people, old and young, all died except this old man and his son.

Author Liu said, good thing can be bad, and bad can turn into a good thing. This thing can never be predicted. And how deep the philosophy is, just like the changeability of life.