David Papineau presents a controversial view of human reason, portraying it as a normal part of the natural world, and drawing on the empirical sciences to illuminate its working. In these six interconnected essays he offers a fresh approach to some long-standing problems. Papineau rejects contemporary orthodoxy that genuine thought hinges on some species of non-natural normativity. He explores the evolution histories of theoretical and practical rationality, indicating ways in which capacities underlying human reasoning have been selected for their biological advantages. He then looks at the connection between decision and probability, explaining how good decisions need to be informed by causal as well as probabilistic facts. Finally he defends the radical view that a satisfactory understanding of desicion-making is possible only within a specific interpretation of quantum mechanics.