On a foggy February morning in Oxford, England, I arrived at the John Radcliffe Hospital, a shiplike nineteen-seventies complex moored on a hill east of the city center, for the express purpose of being hurt. I had an appointment with a scientist named Irene Tracey, a brisk woman in her early fifties who directs Oxford University’s Nuffield Department of Clinical Neurosciences and has become known as the Queen of Pain. “We might have a problem with you being a ginger,” she warned when we met. Redheads typically perceive pain differently from those with other hair colors; many also flinch at the use of the G-word. “I’m sorry, a lovely auburn,” she quickly said, while a doctoral student used a ruler and a purple Sharpie to draw the outline of a one-inch square on my right shin. Wearing thick rubber gloves, the student squeezed a dollop of pale-orange cream into the center of the square and delicately spread it to the edges, as if frosting a cake. The cream contained capsaicin, the chemical responsible for the burn of chili peppers. “We love capsaicin,” Tracey said. “It does two really nice things: it ramps up gradually to become quite intense, and it activates receptors in your sk...