It is a universal experience—the disturbing feeling that someone or something is there when we are alone. Bringing psychiatry, neuroscience, and philosophy to bear upon this enigma, Ben Alderson-Day also considers what can be learned from explorers, mediums, and even robots. Among the intriguing questions considered here are: What is a voice when it isn't heard, and how otherwise do we know or feel that someone is in our presence? Is it a hallucination connected to psychosis, a change in the working of the brain, or something else?