Strong sufficiency is what shows the harm principle is a complete guide to the regulation of liberty, specifying both when regulation is impermissible and when it is required.

But notice that if Mill rejects strong sufficiency then this compromises his one very simple principle. For only strong sufficiency shows that the harm principle is a complete guide to the regulation of liberty, telling us both when regulation is impermissible and when it is required. Even weak sufficiency implies that the harm principle must be supplemented with some other principle, such as the utilitarian principle, in order to determine if regulation is permissible, much less required. Mill’