In his Autobiography (1873) John Stuart Mill reports that in 1821 he discovered that his mission in life was to be "a reformer of the world." Mill was only fourteen years old at the time, and his later intellectual development expanded and modified many of his youthful enthusiasms. But he never abandoned his choice to be an improver of the human lot. Throughout his lifetime Mill used his impressive reasoning and rhetorical skills to support various reforms—philosophical, legal, ethical, educational, and social—and many of his writings are widely recognized as classics of the liberal reform tradition.

Mill was bred, quite literally, to be a social reformer. Born in London, he was the son of James Mill, a disciple of Jeremy Bentham, the brilliant and eccentric Utilitarian philosopher. Bentham advocated many social reforms, most notably in prisons, government, and the law. James and his wife, Harriet Burrow Mill, moved...