The Master and His Emissary takes its title from a story by Nietzsche (McGilchrist, humorously, drops a footnote and says he can't remember where he saw it . . . and apparently the editors at Yale couldn't, either). It goes like this:
A wise spiritual master ruled a small but prosperous country. He was known for his benevolence. His country flourished and began to spread. He needed trained emissaries to help him govern. His most-effective one began to see himself as the ruler and began to undermine his master, eventually usurping him altogether and taking over, ruling the country as a tyrant, which then crumbled.
Here is McGilchrist's lengthier explanation of the fable:
There was a wise spiritual Master who looked after a small community so well that it flourished and grew. Eventually the Master realised that he could not take care of all his people’s needs on his own; more importantly, he realised that there were certain matters that he not only could not, but must not, become involved in, if he were to preserve his overview. He therefore appointed his brightest assistant to go about and do work on his behalf. Though bright, this emissary was not bright enough to know what it was he didn’t know. He became arrogant and resentful of the Master: ‘What does he know?’, he thought. ‘I’m the one that does the real work round here, I’m the one that really knows.’ And so he adopted the Master’s cloak, and pretended to be the Master. The emissary not knowing what it was he didn’t know, the community declined, and the story ends with the ruin of the community, including both the Master and the emissary.
Not ignorance, but ignorance of ignorance, is the death of knowledge. The right hemisphere, however, knows what it is that it must not get involved with. In the story that is why the Master (the right hemisphere) appoints the emissary in the first place.