Mill starts off by limiting the scope of his essay to Civil, or Social Liberty. He writes that this essay will look at what kind of power society can legitimately exert over the individual. Mill predicts that this question will become increasingly important because some humans have entered a more civilized stage of development, which presents "new conditions" under which issues of individual liberty must be addressed.
Mill then turns to an overview of the development of the concept of liberty. In ancient Greece, Rome and England, liberty implied "protection against the tyranny of political rulers," and rulers and subjects were often thought to have a necessarily antagonistic relationship. The leader did not govern by the will of his people, and while his power was seen as necessary, it was also considered dangerous. Patriots tried to limit the leader's power in two ways: 1) They gained immunities called "political liberties or rights." The leader was thought to have a duty to respect these immunities, and there was a right of rebellion if these rights and liberties were infringed. 2) Constitutional checks developed, under which the community or their representatives gained some power of consent over important acts of governance.
Mill writes that eventually men progressed to a point where they wanted their leaders to be their servants, and to reflect their interests and will. It was thought that it was not necessary to limit this new kind of ruler's power, because he was accountable to the people, and there was no fear of the people tyrannizing itself. However, when an actual democratic republic developed (The United States), it was realized that the people don't rule themselves. Rather, the people with power exercise it over those without power. In particular, a majority may consciously try to oppress a minority. Mill writes that this concept of a tyranny of the majority has come to be accepted by major thinkers. Mill, however, argues that society can also tyrannize without using political means. Rather, the power of public opinion can be more stifling to individuality and dissent than any law could be. Thus, he writes that there must also be protection for people against the prevailing public opinions, and the tendency of society to impose its values on others.
The question, then, as Mill sees it, is where and how to limit public opinion's sway over individual independence. There has been very little consensus among nations about the answer to this question, and people tend to be very complacent about their own customs in dealing with dissent. People tend to believe that having strong feelings on a subject makes having reasons for that belief unnecessary, failing to realize that without reasons their beliefs are mere preferences, often reflecting self-interest. Furthermore, on the occasions when individuals do question the imposition of public opinion on social standards, they are usually questioning what things society should like or dislike, not the more general question of whether society's preferences should be imposed on others. Mill also notes that in England there is no recognized principle by which to judge legislative interference in private conduct.
After laying out the major issues, Mill then turns to what he calls "the object of his essay." He writes that he will argue that the only time individuals or society as a whole can interfere with individual liberty is for self-protection. Mill states that the argument that a certain law or public opinion might be for an individual's own good or welfare does not suffice to justify that law or public opinion as a coercive force; coercion by the many toward the individual is only acceptable when an individual poses a threat to others. It is fine to argue with a person about his actions, but not to compel him. Mill writes, "Over himself, over his own body and mind, the individual is sovereign."
Mill notes that the right of liberty does not apply to children, or to "backward" societies. It is only when people are capable of learning from discussion that liberty holds; otherwise the people must be taken care of. Mill also notes that he is not justifying the claim of liberty as an abstract right. Rather, he is grounding it in utility, on the permanent interests of mankind.