FOR MILLENIA IT'S BEEN KNOWN that the human brain is divided into two hemispheres, the left and the right, yet exactly why has never been clear. What purpose this division served once seemed so obscure that the idea that one hemisphere was a "spare," in case something went wrong with the other, was taken quite seriously. Yet the idea that the brain's hemispheres, though linked, worked independently has a long history. As early as the third century B.C., Greek physicians speculated that the brain's right hemisphere was geared toward "perception," while the left was specialized in "understanding," a rough and ready characterization that carries into our own time. In the 1970s and 1980s, the "split brain" became a hot topic in neuroscience, and soon popular wisdom produced a flood of books explaining how the left brain was a "scientist" and the right an "artist." 

Much insight into human psychology can be gleaned from these popular accounts, but "hard" science soon recognized that this simple dichotomy could not accommodate the wealth of data that ongoing research into hemispheric function produced. And as no "real" scientist wants to be associated with popular misconceptions — for fear of peer disapproval — the fact that ongoing research revealed no appreciable functional differences between the hemispheres — they both seemed to "do" the same things, after all — made it justifiable for neuroscientists to put the split-brain question on the back burner, where it has pretty much stayed. Until now.

One popular myth about the divided brain that remained part of mainstream neuroscience was the perception of the left brain as "dominant" and the right as "minor," a kind of helpful but not terribly important sidekick that tags along as the boss deals with the serious business. In his fascinating, groundbreaking, relentlessly researched, and eloquently written work, Iain McGilchrist, a consultant psychiatrist as well as professor of English — one wants to say a "scientist" as well as an "artist" — challenges this misconception. The difference between the hemispheres, McGilchrist argues, is not in what they do, but in how they do it. And it's a difference that makes all the difference. 

Although each hemisphere is involved in virtually everything the brain does, each has its own take on the world, or attitude toward it, we might say, that is radically opposed to that of the other half. For McGilchrist, the right hemisphere, far from minor, is fundamental — it is, as he calls it, "the Master" — and its task is to present reality as a unified whole. It gives us the big picture of a living, breathing "Other" — whatever it is that exists outside our minds — with which it is in a reciprocal relationship, bringing that Other into being (at least for our experience) while it is itself altered by the encounter. The left hemisphere, although not dominant as previously supposed, is geared toward manipulating that Other, on developing means of controlling it and fashioning it in its own likeness. We can say that the right side presents a world for us to live in, while the left gives us the means of surviving in it. Although both hemispheres are necessary to be fully alive and fully human (not merely fully "functioning": a left brain notion), their different perspectives on the outside world often ...