Here he notes that apparitions were studied extensively by the British Society for Psychical Research in the late 19th and early 20th centuries. Basically, there are two categories of apparitions: those of the living and those of the deceased. Thousands of such cases have been reported and researched. The most common of these is known [...]

Here he points out that eighteenth century mesmerists discovered that, during trance, individuals could exhibit two simultaneous and independent states of consciousness. Many variations of this type of experiment were performed over the coming century. In the late nineteenth and early twentieth centuries, clinicians began reporting instances of what was known as multiple personality disorder. [...]