Although "the greatest happiness principle" is Mill's fundamental moral principle, his version of utilitarianism assigns RIGHTS an important role in moral deliberation. He defends rights as an essential ingredient in the promotion of utility.

A right is violated when there is some "wrong done, and some assignable person who is wronged." 

This position is part of his rule utilitarianism (the doctrine that the moral rightness of an act depends on the consequences of people generally following a rule).

Specifically, Mill's theory of JUSTICE has a place for rights, which derive from some duties. Only some moral obligations involve justice, but these "exactly coincide" with the presence of rights and "perfect duties." 

Duties divide into PERFECT and IMPERFECT obligations or duties:

• Perfect duties = those things we must do because of rights
• Imperfect duties = things we should sometimes do, yet no specific actions are  required, and no rights are involved

Examples:

• We have a perfect duty to treat everyone equally and impartially under the law (a police officer or judge can't "look the other way" for members of his or her family).
• We have an imperfect duty to be charitable (we need to do it sometimes, but individuals must choose when, how often, how much, etc.)

Mill proposes that questions of justice arise when one person can claim that another person has a perfect obligation or duty to them, and where other people can interfere to compel the second person to live up to that obligation. (Or several persons, or a group, can claim that some person, persons, or group has a perfect duty to them.) When a perfect duty arises from a rightful claim by one or more specific persons, that duty is associated with "a correlative right." 

In other words, some duties are related to rights, but some duties are not.