Martin Jay tackles a question as old as Plato and still pressing today: what is reason, and what roles does and should it have in human endeavor? Applying the tools of intellectual history, he examines the overlapping, but not fully compatible, meanings that have accrued to the term “reason” over two millennia, homing in on moments of crisis, critique, and defense of reason. After surveying Western ideas of reason from the ancient Greeks through Kant, Hegel, and Marx, Jay engages at length with the ways leading theorists of the Frankfurt School—Horkheimer, Marcuse, Adorno, and most extensively Habermas—sought to salvage a viable concept of reason after its apparent eclipse. They despaired, in particular, over the decay in the modern world of reason into mere instrumental rationality. When reason becomes a technical tool of calculation separated from the values and norms central to daily life, then choices become grounded not in careful thought but in emotion and will—a mode of thinking embraced by fascist movements in the twentieth century. Is there a more robust idea of reason that can be defended as at once a philosophical concept, a ground of critique, and a norm for human emancipation? Jay explores at length the communicative rationality advocated by Habermas and considers the range of arguments, both pro and con, that have greeted his work.