He believed in a moral theory called utilitarianism—that actions that lead to people's happiness are right and that those that lead to suffering are wrong.
Mill's work Utilitarianism, originally published in Fraser's Magazine (1861), is an elegant defense of the general utilitarian doctrine and perhaps remains the best introduction to the subject.
Mill's moral theory of justice and rights is applied to the case of women' subjection to prove that without liberty and without equality human possibilities will never become fully realized and any worthy life is possible for human beings whether they be men or women.