The harm principle

The big idea: Should anyone — the law, the crowd, your neighbours — be allowed to stop you doing something just because it's bad for you?

In On Liberty (1859) John Stuart Mill gives one clear answer, and builds the whole book on it.

Mill, in his own words: "The only purpose for which power can be rightfully exercised over any member of a civilised community, against his will, is to prevent harm to others."

Unpack it: the only reason society may force you to do (or not do) something is to stop you harming other people. That's it. This is the harm principle.

Hold onto this: The word doing the work is others. Harm to other people can justify interference. Harm only to yourself cannot — no matter how strongly anyone disapproves.

Mill's sharpest claim is what the harm principle rules out.

"His own good is not a sufficient warrant": Mill says your own good — physical or moral — is "not a sufficient warrant" for coercing you. Society can argue with you, plead, warn, try to persuade. What it may not do is force you, purely for your own benefit. An adult who understands the risks gets to run them. As Mill puts it, "over himself, over his own body and mind, the individual is sovereign."

Checkpoint — the harm principle: In one line: the only reason to limit an adult's liberty against their will is to prevent harm to others; their own good is never enough. Hold that — next we ask where 'harm to others' actually starts.

The whole rule turns on one line — so Mill draws it carefully.

Go further — higher-level insight: Mill's honest difficulty: almost nothing affects only you. Drink yourself sick and your family suffers too. Mill's reply is to separate mere offence or disapproval (not harm — doesn't count) from a definite injury or broken duty to a specific person (real harm — does count). Naming that distinction is the top-band move on any harm-principle question.