John Stuart Mill was a pivotal thinker of the 19th century who redefined liberal thought and utilitarian philosophy. His landmark essay 'On Liberty' provided the classic defense of individual freedom against state and social interference, introducing the 'harm principle' as the limit of authority. Mill refined the utilitarianism of Jeremy Bentham by emphasizing the quality of pleasures and the importance of individual character development. A vocal advocate for social reform, his work 'The Subjection of Women' was a pioneering text for gender equality. As an economist and Member of Parliament, he influenced modern views on democracy, logic, and human rights, arguing that the ultimate goal of society should be the flourishing of the individual's rational and moral nature.