An old man lived near the northern frontier. One day his horse ran away across the border. His neighbors came to comfort him. This is bad, they said.
How do you know? the old man said.
A few months later the horse returned, bringing a fine wild horse with it. The neighbors came to congratulate him. This is good, they said.
How do you know? the old man said.
His son began riding the wild horse, fell, and broke his leg. The neighbors came to comfort him. This is bad, they said.
How do you know? the old man said.
The next year the army came through the village taking young men to fight in a war. Most of them died. The son, with his broken leg, was left at home.
The story is from the Huainanzi (2nd century BCE).
Good fortune and bad fortune turn into each other. What seems bad now may be the source of what seems good later, and the reverse.
別擔心,塞翁失馬,焉知非福。
别担心,塞翁失马,焉知非福。
Bié dānxīn, sài wēng shī mǎ, yān zhī fēi fú.
Don't worry — the old man lost his horse, who knows it isn't good fortune?