The brain evolved, not to wonder about itself, but to survive. It is the most complex machine known, its inner workings largely a mystery. Over 3 million years, pre-human brains quadrupled in size—mainly in the neocortex, where language and culture reside—to become human brains: “The result was the capacity to take possession of the planet" (107). Brain sciences result from the coming together—the consilience—of biologists, psychologists, and philosophers. The functions of various parts of the brain become obvious when they are damaged; brain-injury patients have been sources of this information. Cell biologists have mapped the details of how neurons connect with one another through dendrites, axons, and synapses, their chemical signals setting off precise chains of stimulation that result in thoughts, feelings, and actions.

Evolutionary biologists have discovered that the forebrain, including the cerebral cortex, is “jury-rigged” in and around older parts of the brain—hindbrain and midbrain—shared with fishes, reptiles, and mammals. Human minds thus contain animal instincts and cunning connected to centers of rationality; thus, our emotions are intimately tied to our reasoning. Despite a vast increase in knowledge about the nervous system, scientists still don’t know “the way the circuits process information to create perception and knowledge" (119).