The author is a brain researcher who has spent his career understanding how the structure of the human brain influences how we process information and, in turn, how we perceive the world around us. In this book, he first takes a deep dive into how our brains work, and especially how the two hemispheres of the brain work together, in different ways, to process information from the environment. The right hemisphere is focused on the world around us, taking in information and making decisions about how to respond. The left hemisphere focuses on logic and analysis, but has only limited interest in the external environment. When the hemispheres work together, the right hemisphere filters incoming information, sends the filtered input to the left hemisphere for analysis, and then uses the results of this analysis to help it make decisions about action.
A tiny aside: I was fascinated by the fact that the center in the left hemisphere that is most closely associated with speech is mirrored in the right hemisphere by the center from which music comes. The anthropological evidence is that our ancestors communicated through musical sound long before they developed language. I was stunned when I read this.
The second half of the book is a reflection on how our brains have shaped civilization. McGilchrist makes the case that, in most of human history, civilization was built on a partnership between the left and right hemispheres in which the left-brain analytic functions provided advice to the right hemisphere, but the right hemisphere was the locus of decision and action. He argues that Western civilization has elevated the status of the left hemisphere in ways that, while making European civilization globally dominant, now endangers our continued success as a species.