Mill's mission in writing On Liberty can perhaps be best understood by looking at how he discussed his work in his Autobiography. Mill wrote that he believed On Liberty to be about "the importance, to man and society, of a large variety in types of character, and of giving full freedom to human nature to expand itself in innumerable and conflicting directions." This celebration of individuality and disdain for conformity runs throughout On Liberty. Mill rejects attempts, either through legal coercion or social pressure, to coerce people's opinions and behavior. He argues that the only time coercion is acceptable is when a person's behavior harms other people--otherwise, society should treat diversity with respect.
Mill justifies the value of liberty through a Utilitarian approach. His essay tries to show the positive effects of liberty on all people and on society as a whole. In particular, Mill links liberty to the ability to progress and to avoid social stagnation. Liberty of opinion is valuable for two main reasons. First, the unpopular opinion may be right. Second, if the opinion is wrong, refuting it will allow people to better understand their own opinions. Liberty of action is desirable for parallel reasons. The nonconformist may be correct, or she may have a way of life that best suits her needs, if not anybody else's. Additionally, these nonconformists challenge social complacency, and keep society from stagnating.
Mill's argument proceeds in five chapters. In his first chapter, Mill provides a brief overview of the meaning of liberty. He also introduces his basic argument in favor of respecting liberty, to the degree it does not harm anybody else. His next two chapters detail why liberty of opinion and liberty of action are so valuable. His fourth chapter discusses the appropriate level of authority that society should have over the individual. His fifth chapter looks at particular examples and applications of the theory, to clarify the meaning of his claims.
Mill's essay has been criticized for being overly vague about the limits of liberty, for placing too much of an emphasis on the individual, and for not making a useful distinction between actions that only harm oneself, and actions that harm others. That said, the essay does provide an impassioned defense of nonconformity as a positive good for society, and an equally impassioned reminder that no one can be completely sure that his or her way of life is the best or the only way to live.