John Stuart Mill was the most influential philosopher in the English-speaking world in the nineteenth century. He wrote extensively on logic and scientific methodology, economics, moral theory, political philosophy, feminism, metaphysics, religion, poetry, history, and botany, as well as on virtually all of the pressing social and political issues of his time; his Collected Works (1963-1990) comprise thirty-one volumes. In the opinion of his contemporary, the eminent philosopher Herbert Spencer, Mill was "the one conspicuous figure in the higher regions of thought." In his writings and his speeches as a member of Parliament, Mill espoused liberal views that shaped the opinions of intellectuals and members of the working class alike. Even in his most abstract and technical philosophical works, such as his writings on logic and scientific method, his goal was the moral improvement of humanity. For Mill, philosophical speculation was not distinct from practical affairs: as a...