Having just defined justice, Mill now turns to the question of whether the sentiment of justice comes from a special, unique tendency of nature, or whether it can be linked to the concerns of utility. Mill argues for the latter.
Mill contends that there are two components to justice. The first is the desire to punish a person who has done harm. This desire comes from the impulse of self-defense, and the feeling of sympathy. All animals have instincts of self- defense. However, unlike animals, humans are capable of sympathizing not only with their offspring, but with all human beings. Furthermore, humans are more intelligent, and thus have a wider range of sentiments and are able to feel that they are a part of a broader community of interests. Justice then, reflects the natural feeling of retaliation, expanded by sympathy and intellect to apply to things that harm society at large. In themselves, these feelings are not moral sentiments. Justice's moral component can be seen rather in the quality of the outrage people feel at an injustice: people can be upset by an injustice not only if it affects them individually, but if it goes against the interests of society at large; this demonstrates a moral concern.
The other component of justice is that there is an identifiable victim who suffers if justice is infringed upon. Mill argues that the idea of a right is not a concept separate from justice, but is rather a manifestation of the other aspects of justice, namely the desire for punishment and the fact that there is an assignable person who has been harmed. A right means that a person has a valid claim on society to protect him in the possession of that right. However, if one wants to know why society should defend this right, Mill argues that the only reason is one of general utility. The sentiment of justice derives its intensity from its link to the animalistic need for retaliation. It gets its moral force from the "impressive" kind of utility that is involved in rights violations--namely the interest of security. People cannot do without security, and require before they can enjoy anything else. Security is so fundamental that its difference of degree as a form of utility becomes a difference in kind. It is so important that it takes on a feeling of absoluteness, of moral necessity.
Mill then observes that if justice exists independent of utility, if it is a standard in its own right that can be understood through introspection, then it is difficult to understand why questions of justice are often so debatable. In fact, there is as fierce a discussion about what is just as there is about what is useful to society, and it is guided by many conflicting ideas. For example, there is a conflict over which acts should be punished, and over the proper apportionment of punishments. In a different arena, there is disagreement over whether people should be paid more for having natural talents, and whether taxes should be graduated, or issued at a flat rate. In fact, the only way to navigate among conflicting claims of justice is to look to the source of its authority, namely, social utility.
This does not mean, however, that there is no difference between the just and the expedient, or that policy is more important than justice. Rather, justice grounded on utility is the chief part, and the most important part, of all morality; it concerns many of the most basic essentials for human well-being. Mill argues that the moral rules that forbid people to harm each other are more important than any rules of policy, rules about how societal affairs should be managed. Furthermore, the preservation of justice preserves peace among human beings. Thus, there is a very strong utility interest in preserving and enforcing justice's dictates.
Mill argues that most of the applications of justice we observe today are simply ways of maintaining the notion of moral rights just discussed. Impartiality is one rule that is partly based in these, but also comes from the very meaning of utility. The greatest happiness principle doesn't have meaning unless each person's happiness, supposed equal in degree, is valued exactly as much as somebody else's. People are seen to have an equal claim to happiness, and an equal claim to the means to happiness. Social inequalities that are not required by expediency are thus seen to be unjust.