Experiences like hearing voices are leading psychologists to question how all people perceive reality. There's a good chance you've hallucinated before. If you've ever felt the buzz of your phone against your thigh only to realize the sensation was entirely in your head, you've had a sensory perception of something that isn't real. And that, according to the psychologist Philip Corlett, is what makes a hallucination. To many, this definition may seem shockingly broad. Hallucinations were long considered the stuff of psychoses or drug trips, not a regular and inconsequential part of life. But Corlett operates on the idea that hallucinations exist within a hierarchy. At the highest level, according to Corlett's collaborator Albert Powers, they would be something like hearing "whole sentences of clearly spoken speech of a being who seems quite real." But, moving further down the line, hallucinations can be far more banal: an imagined text message, a phantom raindrop, a new parent's