Mill sets out to write on 'the nature and limits of the power which can be legitimately exercised by society over the individual'. He suggests that the struggle for liberty has been an ongoing one, noting the Greeks and Romans. He defines liberty this way:
The aim, therefore of patriots, was to set limits to the power which the ruler should be suffered to exercise over the community; and this limitation was what they meant by liberty.
They did this by recognizing 'certain immunities, called political liberties or rights' and forbidding rulers to infringe on them. If a ruler did so, 'specific resistance, or general rebellion, was held to be justifiable'. They also established constitutional checks, requiring such things as votes from representatives as a condition for certain actions.
Having laid down some history, Mill moves on:
The object of this Essay is to assert one very simple principle, as entitled govern absolutely the dealings of society with the individual in the way of compulsion and control, whether the means used be physical force in the form of legal penalties, or the moral coercion of public opinion. That principle is, that the sole end for which mankind are warranted, individually or collectively, in interfering with the liberty of action of any of their number, is sole protection. That the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, it to prevent harm to others.
For, Mill says, "Over himself, over his own body and mind, the individual is sovereign". I don't know if, or to what extent, Mill was controversial in his own day. Today his sentiments are, or should be, familiar to just about everyone. The belief that power should be so contained is at least known, though sadly not always followed.