One of the central concerns of social and political philosophy has been the issue of what limits, if any, there are to the right of the state to restrict the "liberty" of its citizens. Unless one is convinced of the truth of anarchism, there are some actions with which the state may legitimately interfere, and unless one accords no value to personal liberty, there are some actions the state must leave to the discretion of the individual. One of the tasks of political philosophy is to develop and elaborate a theory to determine where these boundaries lie.

In his classical defense of liberalism—On Liberty—John Stuart Mill gave one influential answer to this question. The only reason that could justify the use of coercion against a person is to prevent harm to other people. Such a reason might not be decisive—it might be that the use...