“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.”John Stuart Mill · Despre libertate · 1859 · capitolul I
Power over an individual is justified only to prevent harm to others.
Mill establishes the fundamental principle of liberty: the state or society has no right to interfere in someone's life just because they dislike their behavior. The only exception is when the individual's actions harm others. For example, you cannot be forced not to smoke at home just because it is unhealthy, but you can be stopped from smoking in an enclosed public space where you affect others' health.
Why it matters This principle is the basis of current debates about the limits of liberty, from mandatory vaccination to hate speech.