Mencius said hardship expands what you couldn't do — it's not empty consolation.
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history
From 《孟子·告子下》 'Born of sorrow and death of ease' passage: 所以动心忍性,曾益其所不能 — 'so as to stir the heart, toughen the nature, and increase what one formerly could not do.' Follows the famous list 天将降大任于斯人也.
usage
曾 here is a classical loan for 增 (zēng, to increase) — not 曾 céng 'once.' This reading is a common stumbling point. Almost always quoted with the preceding 动心忍性 and ultimately tied back to 天将降大任于斯人也.