In 1973, psychologist David Rosenhan conducted a controversial and now-famous experiment. Rosenhan instructed eight healthy people to go to emergency rooms around the United States and pretend to hear voices. The voices were not threatening, nor did they instruct these people to act in any dangerous way. Despite that they lacked any other symptoms or psychiatric history, all eight were admitted — some for weeks — and prescribed antipsychotic medications. The doctors who hospitalized them reasoned: “What else besides major mental illness could possibly cause someone to perceive things that aren’t really there?”

“Hallucinations,’’ Oliver Sacks’s 13th book, answers this question.