That question has a very complex and complicated answer, hence why McGilchrist devoted the entire first half of the book to it, I can hardly summarize it here.

That question has a very complex and complicated answer, hence why McGilchrist devoted the entire first half of the book to it, I can hardly summarize it here. You might want to watch the RSA Animated talk by McGilchrist: http://www.youtube.com/watch?v=dFs9WO2B8uI http://www.youtube.com/watch?v=dFs9WO2B8uI If this reminds you of Julian Jaynes, then yes, there exists some similarity, but, to quote Wikipedia: "McGilchrist, while accepting Jayne's intention, felt that Jayne's hypothesis was "the precise inverse of what happened" and that rather than a shift from bicameralism there evolved a separation of the hemispheres." In my opinion, it seems like a better question to ask then consists of asking if what McGilchrist describes holds 'all the way down', and I lack an answer to that.