On Liberty is the English philosopher John Stuart Mill's classic critique of the ethical limits of governmental authority, and remains one of the most influential philosophical treatises ever written. In this 1859 philosophical treatise, the concept of personal liberty went without codification until the publication of this enduring work which applies an ethical system of utilitarianism to society and the state which to this day remains well known and studied. Mill starts with a brief overview of the meaning of liberty and proffers his basic argument in favor of liberty for the individual, to the degree that it harms no one else. The value of liberty of opinion and liberty of action are set forth in two chapters. Emphasizing the importance of individuality versus the "tyranny of the majority," he highlights the positive effects of liberty on all people and on society, as freedom enables progress and prevents social stagnation.

Expounding on society, Mills discusses the appropriate level of authority that society ought to have over the individual and the obligation of that society to protect people who are incapable of exercising their own liberty. Finally, he offers particular examples and applications of his theory, in order to clarify the meaning of his claims.