IN HIS extended essay, On Liberty, published in 1859, the English philosopher John Stuart Mill famously declares, “That the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others.” Mill’s irreducible refutation of tyranny leads him to—I have always felt—one of the most moving passages in literature, in which he extols the moral virtues of Marcus Aurelius, only to register the Roman’s supreme flaw. Mill writes: