Every night, as he fell asleep, a 25-year-old man saw an eye in his ceiling. It was huge and unblinking, and it felt as if the whole world was being sucked into it. He didn't have psychosis, and he wasn't on drugs. But he had taken psychedelics in the past, and he was worried enough to contact Max Greenwald, an M.D.-Ph.D. student at Yale School of Medicine, with a question: Was he developing psychosis?