McGilchrist's theory begins with the fact that our brains are laterally divided, with the two hemispheres differing in their structure and function. This is common knowledge, of course, but he goes well beyond the common and cliched pop psychology division of Left Brain/ Right Brain. In great detail he explains exactly how the two hemispheres differ, and he argues, cogently, a theory of how this division evolved and why. We share the characteristic of a divided brain with many animals: the great apes and other primates, of course, but also many other mammals and birds. McGilchrist argues that in the lives of many animals there are times when they must live in paradox; when two mutually exclusive types of thinking must occur simultaneously: a detailed focus on a particular object and a wider, generalised awareness of environment and context. In response to this need, animals evolved, in effect, two brains, each with its own area of mastery. These two hemispheres are linked, and work together to form a complex and multi dimensioned view of reality.
Mcgilchrist observes that our perceptions shape reality, and the two hemispheres produce two quite distinct worlds which the one organism inhabits simultaneously. He outlines the ways in which, in humans, these two worlds differ:
"the world of the left hemisphere, dependent on denotive language and abstraction yields clarity and the power to manipulate things that are known, fixed, static, isolated, decontecutalised, explicit, disembodied, general in nature, but ultimately lifeless. The right hemisphere, by contrast, yields a world of individual, changing, evolving, interconnected, implicit, incarnate, living beings within the context of the lived world, but in the nature of things never fully graspable, always imperfectly known."
He shows how the interaction of these worlds has produced music and then language. He shows how these worlds are reflected in philosophy and how the shape of the human brain has influenced the structure and working of what we think of as "logical" and "true".
He argues that the right hemisphere is the primary one, but the argument of the book is reflected in its title, which is taken from a short story by Nietzsche in which a wise ruler, wishing to distance himself from the minutiae of governing, appoints an emissary to conduct the day by day business of the country. The emissary comes to be seen as the masters representative and gradually takes on more and more of the master's powers until finally he begins to believe the master is unecessary. McGilchrist says that siomething similar has happened in Western culture, where, over the last few centuries, the Left Hemisphere has begun to dominate the thinking, and therefore the worldview of the West.
The book is divided into two roughly equal parts. Part one of the book explores the nature of the brain and the functions of the two hemispheres. Part two argues the case for the gradual usurpation by the Left Hemisphere of the territory it is not equipped to handle, and the consequences of that for the West, and indeed the world.
This is a profound book, and, at the beginning of the 21st Century, a timely one. McGilchrist's material is informative and his case for a renewed wholeness in human thinking compelling.