Parapsychological phenomena of two types have been described. They may be cognitive, as in the case of clairvoyance, telepathy, or precognition (qq.v.); here one person is believed to have acquired knowledge of facts, of other people's thoughts, or of future events, without the use of the ordinary sensory channels—hence the term extrasensory perception (q.v.; ESP), often used to designate these phenomena. Alternatively, parapsychological phenomena may be physical in character: the fall of dice or the dealing of cards is thought to be influenced by a person's “willing” them to fall in a certain way; or objects are moved, often in a violent fashion, by poltergeists (see poltergeist). The term psychokinesis (q.v.) is often used in this connection. The general term psi has become established to denote all kinds of parapsychological phenomena.

Scientific interest in the subject is of relatively recent origin, but belief in the reality of such phenomena has been widespread since the earliest recorded times. Before the rise of modern science the causation of all complex physical phenomena was very poorly understood, and hence appeals to nonmaterial agencies (ghosts, sorcerers, demons, mythological beings) took the place of a causal,scientific explanation. Even so, there were widespread debates about the reality of phenomena that obviously transcended the bounds of everyday happenings, such as veridical prophecies, as by the oracle of Delphi,or the revival of the dead.

The existence of parapsychological phenomena continues to be a subject of dispute, although societies for the study of psychic phenomena, made up of eminent scientists and laymen, have been in existence for over a century. In 1882 the Society for Psychical Research was founded in London, followed six years later by the founding of a similar society in the United States, partly through the efforts of psychologist William James. Such societies were founded later in most European countries, and active work is carried on, particularly in The Netherlands, France, Italy, Russia, and Japan. Universities have been slower to recognize psychical research as a serious subject for study. The activities of the parapsychological laboratory at Duke University, Durham, N.C., under the American parapsychologist J.B. Rhine from the 1930s to the 1960s attracted considerable interest. A department of psychical research later was opened at the University of Utrecht under W.H.C. Tenhaeff.

One of the reasons for interest in psychical research in the last half of the 19th century wasthe rise of the spiritualist movement that grew out of the acceptance of spirit communication as real and the use of this as the basis of a new religion. Some of the early psychical researchers were also spiritualists, as, for example, British spiritualist F.W.H. Myers and the British physicist Sir Oliver Lodge. Other psychical researchers (such as the French physiologist Charles Richet) accepted paranormal activity as real but rejected the spiritualist explanation, while others were not committed to either view.

Discussion about parapsychological phenomena has sometimes assumed emotional overtones, unsuitable to scientific discipline, and outspoken but contradictory opinions are still frequently voiced. Believers and nonbelievers in psi may base their belief or disbelief on what they consider to be the scientific evidence, on their personal experiences, or on some larger system of attitudes and values into which ESP does or does not fit. When such extreme and contradictory views are widely held, it is almostcertain that the evidence is not conclusive either way and that confident conclusions are unlikely to be supported by a survey of all the known facts.