Do the spirits of the dead influence the living? Could some cases of mental illness be caused by spirit obsession or so-called possession?For years, psychiatrists and parapsychologists have grappled with the bizarre hallucinations and delusions of the mentally disturbed and some have wondered if there might lurk some level of paranormal perception-A reality that goes far beyond the five senses.The possibility first came to light early in the twentieth century, when Frederic Thompson a Massachusetts jeweller, claimed he was possessed by the recently deceased spirit of R. Swain Gifford, a celebrated landscape painter. One day Thompson suddenly began painting oils and drawing sketches in the style of Gifford, even though he claimed he had no formal training. Thompson's paintings and sketches matched unfinished and never before exhibited work left by Gifford when he died, while others represented Gifford's remote island home.Professor James Hyslop, a psychologist at Columbia who had recently resigned his faculty position to run the American Society for Psychical Research (ASPR) took on the case and spent years establishing that possession might be the root cause in some cases of both madness and multiple personality.Others followed in Hyslop's footsteps: Dr Titus Bull, a neurologist claimed he cured patients by exorcizing them with the help of a Spiritualist medium; Dr Elwood Worcester, a social pioneer whose attempts to unite religion with psychology led him to confront cases of spirit possession; and Dr Walter Prince, a clergyman-turned-psychologist who cured paranoia by treating patients with exorcism.This is their story.