This is quite a book. Its sheer heft is daunting, its central claim bold and sweeping, its data relentless. While the planet goes “cycling on according to the fixed law of gravity,” Steven Pinker argues, the human species has been progressing. We have reduced the fear of violent death for an ever-greater proportion of the population across the centuries. Pinker argues that every sort of violence has declined, from interpersonal cruelty to interstate war, beginning toward the end of the medieval period and extending to today. He then identifies the causes of decline so that we can know what can extend the trend into the future. This will allow us, as he puts it, to “obsess not just over what we have been doing wrong but also over what we have been doing right” (p. xxvi).