The central figure of 19th-century English Classical Liberalism and the thinker who made Utilitarianism philosophically respectable by widening it past the narrow hedonism of his godfather Jeremy Bentham. Raised in a famously rigorous home-schooling regime by his father James Mill, he suffered a mental crisis in his twenties which he credited to an upbringing that neglected feeling — a crisis resolved, in part, by reading Wordsworth.
On Liberty (1859) is his most enduring statement: the "harm principle" — that the only justification for coercing an individual is to prevent harm to others — remains the touchstone of liberal argument about free speech, lifestyle, and state power. Utilitarianism (1861) distinguishes higher and lower pleasures, trying to rescue the doctrine from the charge that it reduces ethics to pig-satisfaction. The Subjection of Women (1869), written with and deeply shaped by his wife Harriet Taylor Mill, is one of the foundational texts of liberal feminism.
Late Mill drifted toward a cautious socialism, worried that unregulated capitalism threatened the very individuality his liberalism prized.