Bartlett’s Critique of Impure Reason is a tour de force of philosophical analysis. Historic in scale and far-reaching in scope, this important and massive treatise offers a penetrating critical appraisal of concepts that form the basic vocabulary of philosophical problems. This groundbreaking work focuses on reference as the most conceptually basic means in terms of which we may understand the limits beyond which our concepts cease to possess possible meaning. These inevitable and inescapable boundaries of human thought Bartlett calls ‘horizons’. Step by logical step, the treatise seeks to show that a recognition of these conceptual horizons brings with it a new and revisionary understanding of a wide range of philosophical problems. The book is as fascinating as it is profound.