Utilitarianism is a moral theory based on the principle that "Actions are right in proportion as they tend to promote happiness, wrong as they tend to produce the reverse of happiness."
Mill sees happiness as humans' primary desire; hence, it is the basis for morality and justice. In defense of utilitarianism, Mill defines and tries to validate his position by relating justice and utility and arguing that happiness is the foundation of justice. While the whole argument can be presented in many ways, Mill tried to vindicate Utilitarianism from accusations such as: not protecting individual rights, measuring everything in life with the same standard, and over simplifying happiness.