A man standing by a spring of clear, sweet water and cursing it. While the fresh water keeps on bubbling up. He can shovel mud into it, or dung, and the stream will carry it away, wash itself clean, remain unstained. (Hays translation)

Suppose that men kill thee, cut thee in pieces, curse thee. What then can these things do to prevent thy mind from remaining pure, wise, sober, just? For instance, if a man should stand by a limpid pure spring, and curse it, the spring never ceases sending up potable water; and if he should cast clay into it or filth, it will speedily disperse them and wash them out, and will not be at all polluted. How then shalt thou possess a perpetual fountain? By forming thyself hourly to freedom conjoined with contentment, simplicity and modesty.

VIII, 51