The sentence is the limit of the harm principle: society may interfere with you to prevent harm to others, and not to prevent harm to yourself, and not because your opinions are unpleasant. Mill had been educated by his father as an experiment and suffered for it. The sovereignty he claims is hard, adult, and incomplete — he did not extend it cleanly to the colonized. The sentence remains the best short statement of a liberal boundary, including against the liberals who came after.