Some years ago, scientists in Switzerland found a way to make people hallucinate. They didn’t use LSD or sensory deprivation chambers. Instead, they sat people in a chair and asked them to push a button that, a fraction of a second later, caused a rod to gently press their back. After a few rounds, the volunteers got the creeping sense of someone behind them. Faced with a disconnect between their actions and their sensations, their minds conjured another explanation: a separate presence in the r