This text takes the form of a conversation between the author and imaginary listener. It journeys from the story of Phineas Gage, the 19th-century case of behavioural change that followed brain damage, to the contemporary recreation of Gage's brain; and from the doubts of young neurologist to a testable hypothesis concerning the emotions and their fundamental role in rational human behaviour. Drawing on his experiences with neurological patients affected by brain damage, Damasio shows how the absence of emotion and feeling can break down rationality. In the course of explaining how emotions and feelings contribute to reason and to adaptive social behaviour, he also offers a perspective on what emotions and feelings actually are: a direct sensing of our own body states, a link between the body and its survival-orientated regulations, on the one hand, and consciousness, on the other. Damasio concludes that human organisms are endowed from the very beginning with a passion for making choices, which the social mind can use to build rational behaviour. Hearts have reasons that reason should know of and use.