John Stuart Mill

Mill was the son of the Benthamite philosopher James Mill, raised on a punishing curriculum: Greek at three, Latin at eight, logic at twelve, political economy at thirteen. At twenty he had a famous nervous breakdown, recovered partly through reading Wordsworth, and spent the rest of his life softening his father's grim utilitarianism into something humane.

In utilitarianism, the right action is the one that produces the greatest happiness for the greatest number. Mill refined Bentham by distinguishing higher from lower pleasures — "better to be Socrates dissatisfied than a fool satisfied." In On Liberty he made the classic liberal argument for freedom of thought, speech, and lifestyle: the only legitimate use of state power against an individual is to prevent harm to others. Coauthored with his wife Harriet Taylor, The Subjection of Women was one of the first great philosophical arguments for the equality of the sexes.

Almost every modern democratic argument about free speech, freedom of conscience, or the role of the state is downstream of Mill.