Jacob Sullum looks at the latest government survey on mental health. The opening paragraphs are entertaining and to the point. According to a new government-sponsored survey, most Americans qualify for a psychiatric diagnosis at some point in their lives. Trying to explain how so many of us became mentally ill, The New York Times offered a history lesson that reminded me of an old “Saturday Night Live” sketch. In the sketch, Steve Martin plays Theodoric of York, a medieval barber with a patient whose condition has not improved despite a bloodletting, a sheep’s-urine-and-staghorn poultice, and a night buried in the marsh up to her neck. “Medicine is not an exact science,” Theodoric tells the girl’s mother, “but we are learning all the time. Why, just 50 years ago, they thought a disease like your daughter’s was caused by demonic possession or witchcraft. But nowadays we know that Isabelle is suffering from an imbalance of bodily humors, perhaps caused by a toad or a small dwarf living in her stomach.” To provide context for the government’s mental health survey, the Times told a similarly inspiring story of science replacing superstition. In the old days, it explained, “gamblers and drinkers, the excessively impulsive or rebellious, [and] the sexually promiscuous . . . were considered sinners, deviants or possessed,” while “those who denied themselves food or comfort, or who prayed or performed ritual cleansing repeatedly, often struck others as especially pious.” But “as science gradually displaced religion,” the Times continued, “such behavior was increasingly seen in secular, diagnostic terms.” Hence “excessive fasting became anorexia,” and “ritualized behavior was understood as compulsive, or obsessive-compulsive.” This is the sort of progress that might impress Theodoric of York, but does it bring us any closer to the truth? This review in Reason by Jacob further explores the issue of defining mental illness.