Some of you may already be thinking: not the old chestnut about the hemispheres again, surely? Despite frequent acknowledgment by many leading neuroscientists that there do seem to be fundamental differences between the hemispheres, and despite some tantalising glimpses, scientists have largely given up trying to put their finger on what these might be, piece after piece of information showing that every conceivable activity – language, visual imagery, and all the things we thought in the past distinguished right from left – is served by both hemispheres, not just one. The problem is that we generally look at the brain as having “functions”, and if you do that, sure enough, those functions are shared by both hemispheres. But if you look, not at what the brain does, as if it were just a machine, but at how – in the sense of “in what manner” – it does it, as if it were part of a living person, some very important differences start to emerge, and a picture begins to take shape that tells us some astonishing things about ourselves and our world. My view is that the relationship between the hemispheres, like that of the master and his emissary in the story, is not symmetrical. Each needs the other; each has an important role to play.