John Stuart Mill

The only rightful power over another person is to prevent harm.

In their words

“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.”

Mill's On Liberty set out the harm principle: coercion is justified only to prevent injury to others, and over opinions, tastes and conduct the person herself is sovereign, even when she is a fool. He argued that suppressing opinion injures the majority more than the eccentric, that custom deadens character, and that social tyranny arrives before legal tyranny does.