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You possess a prodigious brain. It contains around one hundred thousand million nerve cells, with perhaps a thousand times as many interconnections between them. It is three times the size that would be predicted for an ape of our proportions. The striking process of encephalisation that has occurred since our ancestral line departed from that of the chimp, five million years ago, is undoubtedly one of the cornerstones of our humanity. 

But while size matters, a line of research originally launched by 19th-century neurologists, including Pierre Broca in France and Carl Wernicke in Germany, has pointed to a second key principle of human brain evolution — the lateralisation of brain function. 

The apparent symmetry of the brain's paired hemispheres is deceptive: our preference for using our "dominant" hand, usually of course the right, for skilled tasks, turns out to reflect a profound division of labour between the two sides of the brain. 

For most of us, the left hemisphere, controlling the right hand, takes the lead in comprehending and expressing language, and in praxis, the control of skilled action, while the right hemisphere leads in perception and spatial awareness. This specialisation of function within the hemispheres has allowed an increase in the efficiency of neural processing which has greatly augmented our brain power — or such is the textbook view. 

Iain McGilchrist's remarkable book extends this received wisdom with a hugely ambitious, absorbing and questionable thesis: the two hemispheres have radically contrasting personalities; that they live in a state of creative tension, sometimes declining into open war; and that their struggle for supremacy provides the key to understanding the major cultural movements of human history. 

Every so often in neurology, one encounters cases that point beyond the standard view of hemisphere differences in the direction McGilchrist pursues. 

A young man with a stroke near the front of the left hemisphere plunges abruptly into suicidal depression — despite the near complete loss of language, he manages to convey his state of mind with graphic intensity. His shift of mood seems to reflect some loss of balance, an unopposed force. Following surgery to the right hemisphere, a second patient loses the ability to "get" the gist of conversation — there is no problem with language, but there seems to be a grave problem with sense. An elderly man with a form of dementia that affects the left hemisphere develops a passion for art: it is as if a suppressed capacity has been released. Clinical experience indeed sometimes hints that the two sides of the brain are in tension, though hints falls short of any kind of proof.

Some of these hints are being more systematically explored, using the novel techniques, like functional Magnetic Resonance Imaging (fMRI), that allow us to visualise the brain in action — this exploration is work in progress. McGilchrist builds up his image of the two hemispheres by way of a series of contrasts. While he claims impartiality, it is clear where his sympathies lie. But he emphasises, surely rightly, that most human functioning, and all major human achievement, require a collaborative effort between the hemispheres. Here are some of the contrasts in question. The right hemisphere, the Master, is engaged, empathetic, receptive, intuitive, metaphorical, humorous, particular, musical, holistic; the left, the Emissary, is detached, rational, acquisitive, conceptual, literal, straight-laced, abstract, verbal, analytic.

In McGilchrist's view, while the right hemisphere grounds our experience, the left dissects it. The right hemisphere is at home in our "embodied existence" in art and in religion. The left is at home in designing tools with which to master and understand the world. The left hemisphere treats us and our environment as an assemblage of machines; the right hemisphere treats us as people. The left systematises while the right empathises. 

The cardiologist and scientist John Martin wrote: "I take apart their insides, discover the insides of their insides, until I know the atoms of the molecules that make the cells stick. But where is man desiring beauty?" McGilchrist would undoubtedly answer, "In the right hemisphere!" This view has precedents: when asked to define the role of the right hemisphere, a distinguished researcher on hemisphere differences, John Cutting, answered simply: "It is life, life itself." McGilchrist goes further: the left hemisphere is the "Berlusconi of the brain", with a "Gorgon stare", while the right hemisphere attends to "the body, the spirit and art", which are collectively "the vehicles of love". 

The contrasts drawn between the hemispheres in the first part of this long book are used, in part two, to interpret cultural history in terms of the alleged continual tension between the hemispheres. At the risk of caricature, McGilchrist views several periods and movements in human history as expressions of fruitful collaboration between left brain and right, while others reflect the imperialistic and naïve ambition of the left hemisphere, the Emissary, to take charge. Ancient Greek culture, the Renaissance, the Romantic movement and some — somewhat few — aspects of contemporary culture fall into the first category. The Roman Empire, the Reformation, the Enlightenment, the Industrial Revolution and much of modernism are to be found in the second. He believes we have recently entered a dangerous period of left hemisphere ascendancy, characterised by a proliferation of "small, complicated rules".

This is an unusual book, reflecting the unusual trajectory of its author. Three-times elected Fellow of All Souls, McGilchrist has transformed himself from English literature don into psychiatrist, so travelling from the arts to the sciences, from right hemisphere to left-and, with this book, back again. (This three-part sequence, incidentally, is the cerebral itinerary that he recommends for experience in general.) 

McGilchrist is immensely erudite. He writes with great clarity, and while the book develops an argument it is also a treasure chest of fascinating detail and memorable quotation. Its thesis is profoundly interesting: most readers who enter here with time to spend will be richly rewarded. On occasion the book overreaches itself, and-as ever-some of its factual claims may be wrong: the degree to which the hemispheres are really in conflict is open to question (and investigation) and McGilchrist's view of schizophrenia, one of the book's many themes, is only one of many. 

But the effort to make sense of the totality of our lives in terms of brain function is exhilarating and worthwhile. On one final point I am doubtful: early in the course of this tale of two brains, McGilchrist writes: "It has been said that the world is divided into two types of people, those who divide the world into two types of people, and those who don't. I am with the second group." I am not sure I believe him. 

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