Parapsychology, also known as psychical research, the investigation of alleged phenomena and abilities that apparently cannot be explained by conventional scientific theories. Parapsychologists divide paranormal phenomena into two classes. Extrasensory perception (ESP) is defined as the ability to acquire information without the use of the senses, and includes telepathy (when the information comes from another person); clairvoyance (when the information comes from a distant object or event); and precognition (when the information is about the future). Psychokinesis (PK) is the supposed ability to influence distant objects or events without using known physical forces; this is divided into macro-PK (for example, moving large objects or bending spoons) and micro-PK (for example, influencing computer systems or random number generators). "Psi" is a general term covering them all. Also studied are ghosts, poltergeists (sometimes considered as a form of PK), apparitions of the dying or dead, mediumship, and out-of-body experiences (in which people feel as though they have left their bodies and can see and move without them). Parapsychologists tend to concentrate on ESP and PK, and more often carry out laboratory experiments, while psychical researchers tend to concentrate on spontaneous cases and phenomena suggestive of human survival after death. History of Psychical Research Spiritualism, which began in the 1840s in North America, was the foundation for psychical research. Spiritualist mediums claimed to contact the spirits of the dead through mental mediumship (conveying messages from the "other side", often in a trance state) and physical phenomena (such as table-tapping or tilting, levitation, and writing on closed slates). It was claimed that a materialized spirit, made from a substance which acquired the term ectoplasm, exuded from the medium's body. Since these phenomena appeared to challenge the laws of physics, some scientists tried to test them. Michael Faraday experimented with table-tilting; in 1853 he concluded that it was due entirely to unconscious muscular action by the sitters. William Crookes took
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