"Fortune lashes and mangles us: well, let us endure it: it is not cruelty, it is a struggle, in which the oftener we engage the braver we shall become. The strongest part of the body is that which is exercised by the most frequent use: we must entrust ourselves to fortune to be hardened by her against herself: by degrees she will make us a match for herself. Familiarity with danger leads us to despise it. Thus the bodies of sailors are hardened by endurance of the sea, and the hands of farmers by work; the arms of soldiers are powerful to hurl darts, the legs of runners are active: that part of each man which he exercises is the strongest: so by endurance the mind becomes able to despise the power of misfortunes." --Seneca ("Of Providence")
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Related quotes:
"There were two vices much blacker and more serious than the rest: lack of persistence and lack of self-control....Endure and Renounce." --Epictetus
"By endurance we conquer." --Shackleton Family Motto (“Fortitudine Vincimus”)