Power can be rightfully exercised over anyone, against their will, only to prevent harm to others — never for their own good alone.
Mill laid this out in On Liberty (1859) as the one liberty-limiting principle he was willing to defend without qualification: not offense, not immorality, not even the individual’s own welfare — only demonstrable harm to someone else justifies coercion. It remains the reference point every subsequent argument about the limits of paternalistic law has to either use or explicitly reject.
Joel Feinberg’s four-volume The Moral Limits of the Criminal Law (1984–88) is the standard modern treatment, separating the harm principle from three weaker cousins — the offense principle, legal paternalism, and legal moralism — and testing where each one actually holds up. The principle also has real critics: legal scholars have argued its practical force has eroded as “harm” gets stretched to justify restrictions Mill likely wouldn’t have countenanced.
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Source: Mill, J.S. On Liberty, 1859. Feinberg, J. The Moral Limits of the Criminal Law, Vol. 1: Harm to Others, 1987.