On Liberty

John Stuart Mill’s 1859 essay argues that coercion over a person is justified only to prevent harm to others. It defends individuality, freedom of thought, and open discussion.

Harm principle: The principle that coercion over a person is justified only to prevent harm to others. It states the essay’s central test for limiting individual freedom.
Marketplace of ideas: The view that open competition among opinions helps expose error and advance truth. Mill argues that unrestricted debate gives beliefs a chance to withstand challenge.
John Stuart Mill: A nineteenth-century British philosopher, political economist, and member of Parliament. Mill wrote and published the essay as a defense of individual liberty.
Social contract: A theory that political authority derives from an agreement, explicit or hypothetical, among individuals. Mill’s argument focuses on limits to coercion rather than deriving political authority from consent.
Individuality: The distinctive development and expression of a person’s character and way of life. Mill treats individuality as a central element of human flourishing.
Fallibilism: The position that human beliefs may be mistaken and should remain open to correction. The possibility of error supports Mill’s case against silencing dissent.
Harriet Taylor Mill: An English philosopher and writer whose intellectual partnership with Mill shaped his work. Mill credited Taylor’s influence on the ideas and composition of the essay.
John Locke: A seventeenth-century English philosopher associated with natural rights and government by consent. Locke’s rights-based defense of liberty differs from Mill’s appeal to utility.
Freedom of speech: The liberty to express opinions without government suppression or punishment. The essay applies its defense of liberty especially forcefully to discussion.
Dead dogma: A belief repeated without understanding its reasons or engaging with objections. Mill says even true opinions become lifeless when held without contest.

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