Ernest Gellner (1925–95) was a multilingual polymath and a public intellectual who set the agenda in the study of nationalism and the sociology of Islam. Having grown up in Paris, Prague, and England, he was also one of the last great Jewish thinkers from Central Europe to experience directly the impact of the Holocaust. His intellectual trajectory differed from that of similar thinkers, both in producing a highly integrated philosophy of modernity and in combining a respect for nationalism with an appreciation of the power of modern science. Gellner was a fierce opponent, in private as well as in public, of such contemporaries as Michael Oakeshott, Isaiah Berlin, Charles Taylor, Noam Chomsky and Edward Said. As this definitive biography shows, he was passionate in the defense of reason against every form of relativism—a battle that his intellectual inheritors continue to this day.