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It’s not true that one part of the brain does reason and the other does emotion, says renowned psychiatrist Iain McGilchrist. The purpose of the frontal lobes is to inhibit the rest of the brain, to stand back from the moment, which enables us to outwit and deceive – but also to empathise. This division has had profound consequences for human society.

Dr McGilchrist explains:

‘Humans have large frontal lobes, which enable them to stand back from experience: this puts the hemisphere division to new use.  For purposes of manipulation, the brain needs a relatively simple map of the world which enables it to be efficient in getting hold of things: denotative language and the ability to grasp with the hand are its tools in this representation and manipulation of the world, and they are controlled, as one might expect, from the left hemisphere.

'All the rest, the ability to pick up the complexity of experience and take the broadest view, goes on in the right – which also means that it sees us, not as atomistic, distinct entities in competition with one another, as the left hemisphere must, but as interconnected, interdependent entities. Empathy, social understanding, humour, metaphor, more subtle emotional understanding, the appreciation of individuals, the reading of faces, and much else goes on in the right hemisphere.

'Fascinatingly, there is clear evidence that the left hemisphere alone codes for machines and tools – even in left-handers, who would be using their right hemisphere to use tools and build machines in daily life. 

'Overall, it seems that the right hemisphere sees and knows far more than the left hemisphere, but does not have the left hemisphere’s tools for asserting its point of view: denotative language and serial analysis.  Applying them achieves something very important, certainly, but it is also incompatible with seeing the whole. Hence the need for separation of the two realms of thought and experience (the principle function of the corpus callosum is to inhibit).  But the relationship between them is asymmetrical, as is the brain itself.

'The first appreciation of anything comes to us via the right hemisphere, and the ultimate understanding of it in context does so also. Some very subtle research by David McNeill, amongst others, confirms that thought originates in the right hemisphere, is processed for expression in speech by the left hemisphere, and the meaning integrated again by the right (which alone understands the overall meaning of a complex utterance, taking everything into account).

'More generally, I would see the left hemisphere as having an intermediate role: it ‘unpacks’ what the right hemisphere knows, but then must hand it back to the right hemisphere for integration into the body of our knowledge and experience.

'The trouble is that the left hemisphere’s far simpler world is self-consistent, because all the complexity has been sheared off – and this makes the left hemisphere prone to believe it knows everything, when it absolutely does not: it remains ignorant of all that is most important.’

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