On Liberty

On Liberty (1859) is John Stuart Mill's most influential book and the single most cited defence of individual freedom in the English language. Mill wrote it jointly with his wife Harriet Taylor Mill in the years just before her death, and the argument is built around what later commentators have called the harm principle: that the only purpose for which power can rightfully be exercised over any member of a civilised community, against his will, is to prevent harm to others.

From that one principle Mill develops, across five short chapters, his celebrated defences of freedom of thought and discussion, of individual self-development against the levelling pressure of conformity, and of the strict limits on what laws and customs may legitimately impose. The book has remained the founding modern statement of liberal individualism — read across the political spectrum, taught in every introductory course in political philosophy, and continuously argued with for a century and a half.