“The neuroscientist Paul MacLean, who developed the three-part description of the brain that I’ve used here, compared the relationship between the rational brain and the emotional brain to that between a more or less competent rider and his unruly horse. As long as the weather is calm and the path is smooth, the rider can feel in excellent control. But unexpected sounds or threats from other animals can make the horse bolt, forcing the rider to hold on for dear life.”