Published in 1859, John Stuart Mill’s On Liberty is one of the most celebrated defences of free speech ever written. In this elongated essay, Mill aims to defend what he refers to as “one very simple principle,” what modern commentators would later call the harm principle. This is the idea that people should only be stopped or restrained from acting when their conduct may harm another individual.

Mill states that “the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others.” However, Mill’s simple principle is not the only focus of On Liberty. He also discusses the struggle between liberty and authority, the importance of individuality, the limits of state authority, and the practical application of the harm principle. It is a small yet dense essay with many questions about how a free society ought to treat its citizens.

Mill’s answers provide the bedrock of what we today call liberalism.