John Stuart Mill developed *On Liberty* with his wife Harriet Taylor Mill over many years and published it in 1859. After Harriet's sudden death in 1858, Mill printed the book without changing a single sentence; the dedication at the front calls her 'the partner and inspirer of all my thought'.

The book's core formula is the 'harm principle': the only legitimate ground on which the state or society may interfere with the action of an individual is harm to others. If a person is harming only themselves — losing their health, their money, or their reputation — society may warn or persuade them, but not coerce. 'Over himself, over his own body and mind, the individual is sovereign.'

Mill applies the principle to three domains: freedom of thought and expression; freedom of taste and life-style; freedom of combination (association). His defence of free expression is especially strong: even the worst idea must be freely expressible, because that is the condition under which true ideas remain alive. A truth never challenged becomes 'dead dogma'. With this argument Mill lays out one of the foundational defences of modern liberal democracy.

Mill's antagonist is not only despotic government but the 'tyranny of the majority'. Democracy alone does not guarantee liberty; the majority can press the minority through social pressure, exclusion, and moral judgement — more effectively than laws. That warning has framed debates about populism and social engineering for the 150 years since.

The book is short — about 160 pages — but dense. Bertrand Russell, Isaiah Berlin, and John Rawls all worked in the line it opened. *On Liberty* is read again in every modern argument over the limits of the individual and the state.