Mill writes that if a person causes harm to others actively or inactively, it is appropriate for society to condemn him legally or through general disapprobation. Individuals can even be compelled to do good for other people, such as to save someone's life, because to do otherwise would be to cause evil to another person. In contrast, society only has an indirect interest in what a person does to himself or to other freely consenting people.
Mill divides the appropriate sphere of human liberty falls into three categories, claiming that any free society must respect all three. First, there is the domain of the conscience, and liberty of individual thought and opinion. Second, there is planning one's own life, and the liberty of tastes and pursuits. Third, there is the liberty to unite with other consenting individuals for any purpose that does not harm others. These liberties reflect the idea that true freedom means pursuing one's own good in one's own way, as long as it does not prevent others from doing the same. These ideas directly contradict society's increasing tendency to demand conformity, and unless moral conviction turns against this tendency, the demand for conformity will only increase.
Mill's introduction is one of the most important parts of his essay, as it contains the basic structure of his argument, as well as some of his major presuppositions. Mill describes civilization as a struggle between society and the individual about which should have control over the individual's actions. Mill sees the world as tipping toward a balance in which society, through laws and public opinion, has far more power over the actions and thoughts of an individual than an individual has over himself. Mill rejects this status, arguing that society should have control over only those actions that directly affect it, or those actions that harm some of its members. Mill argues that an individual harming himself or acting against his own good provides insufficient reason for others to interfere. His essay will be a description of why this is the case.
It is important to note that in rejecting social interference with individual thought and activity, Mill is not just writing about laws, but also about "moral reprobation." An individual or group cannot rightly punish a person's behavior by, for example, treating him as an enemy, if his actions only affect himself. In rejecting the legitimacy of coercive opinion, Mill drastically broadens the scope of his claims. It is worth paying attention in later chapters to why Mill is so critical of public disapproval of behavior, and to the avenues that Mill does leave open for people to express disapproval of actions they dislike.
The idea of progress is integral to Mill's essay, and this chapter reflects a few of his ideas on the subject. Mill believes that individuals and society as a whole can improve themselves. Fitting with this idea, he considers different societies to exist on a clear hierarchy of value: barbaric societies are childlike, without the necessary tools of self-government. They must be governed like children, so that they can eventually become capable of exercising their liberty. Yet while Mill considers progress and civilization to be definite goods, he also expresses concern that with progress comes conformity. In later chapters he will try to show that such conformity could undermine further individual and social improvement.
In this introduction, Mill explicitly calls his justification of liberty utilitarian. In doing so, he says outright that his defense of liberty will not be based on natural rights, such as those proposed by Locke, or on metaphysical claims, such as those proposed by Kant. Rather, Mill bases his argument on what is best for mankind, and in doing so suggests that his arguments will show the individual and social benefits of human liberty. In later chapters, it is worthwhile to examine when and how Mill makes broad utilitarian arguments for liberty, and to similarly look for instances when Mill resorts to non-utilitarian arguments.