Good luck and bad turn into each other without warning, so neither is to be rejoiced at or grieved over yet. The story is a Han parable in the 淮南子: an old man by the frontier fort loses his horse, gets it back with another, sees his son break a leg on the new one, and sees that leg keep the son out of the war, declining to react at each turn.
人間 is read ningen, as every Edo witness has it; jinkan is a learned modern choice.
The seven characters are the seventh line of a poem by the Yuan Zen monk 晦機元熙 (Huiji Yuanxi), about 1300, 寄徑山虛谷陵和尚, and a Japanese dictionary of 1444, the 下学集, already names him as its poet. A proverb book of 1701 gave the couplet to Su Dongpo, and proverb books went on doing so into the 1930s, though a Confucian scholar's essay, printed in 1763, had set the monk back. Behind the line stands a much older Chinese parable, the 淮南子's second-century-BC chapter 人間訓, which the monk's couplet distills rather than retells.
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