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 workings. In these six interconnected essays, he offers a fresh approach to some long-standing problems. Papineau rejects the contemporary orthodoxy that genuine thought hinges on some species of non-natural normativity. He explores the evolutionary 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 decision-making is only possible within a specific interpretation of quantum mechanics. By placing the subject in its scientific context, Papineau shows how human rationality plays an explicable role in the functioning of the natural world.