Prehistoric hunter-gatherers encountered fewer people in a lifetime than we would on a single day walking down New York’s Fifth Avenue. Though their world of isolated clans shaped our natures, we live in a world where our most trivial deeds can affect unknown millions on the other side of the globe. Kwame Anthony Appiah, an Anglo-Ghanaian philosophy professor based at Princeton, proposes two principles to enable us to cope with this situation. We are, he asserts, responsible for every other human being. This may seem vapid – the kind of pious over-statement beloved of international organisations. But Appiah takes it seriously, and tries to impose realistic limits on an apparently open-ended duty.