If you touch something hot, it hurts. You snatch your hand away from the hot thing immediately. Obviously. But what is really happening, biologically -- and emotionally? In Understanding Pain, Fernando Cervero explores the mechanisms and the meaning of pain. You touch something hot and your brain triggers a reflex action that causes you to withdraw your hand, protecting you from injury. That kind of pain, Cervero explains, is actually good for us; it acts as an alarm that warns us of danger and keeps us away from harm.

But, Cervero tells us, not all pain is good for you. There...