After years of cultivation, her mind was as still as a placid old well.
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history
From Bai Juyi's (白居易) Tang-dynasty poem 《赠元稹》: '无波古井水,有节秋竹竿' — 'a still old well's water, an upright autumn bamboo'. Bai used the image to praise his friend Yuan Zhen's steady, principled character.
memory
Picture an ancient well with no wind, no ripples — perfectly still water. That's the mind of someone unmoved by external events.