John Stuart Mill was the 'Saint of Rationalism' according to William Gladstone. But in the first definitive biography of Britain's greatest public intellectual, Richard Reeves reveals Mill as a passionate man of action: a philosopher, radical MP and reformer who profoundly shaped Victorian society and continues to illuminate our own. Mill's activism started early: walking across St James' Park in 1823 at the age of seventeen, he discovered a recently murdered baby. London was full of families who could not afford another child. Mill took action, touring working-class districts distributing a pamphlet describing forms of contraception. He was arrested and jailed for promoting obscenity. By this time, Mill had already embarked on a successful career in the East India Company, wowed the Cambridge Union and completed a prodigious course of learning. But his achievements had only just started. This youthful activist would become the highest-ranked English thinker of the nineteenth century, the author of the landmark essay On Liberty and one of the most passionate reformers and advocates of his revolutionary, opinionated age. As a journalist he fired off a weekly article on Irish land reform as the people of that nation starved, as an MP he introduced the first vote on women's suffrage and fought to preserve free-speech and, in his private life, pursued for two decades a love affair with another man's wife. Mill also fearlessly tackled issues such as the regulation of gambling, prostitution and alcohol, as well as reform of the House of Lords and domestic violence.