The term ‘psi‘ (psychic ability or psychic functioning) has been adopted to describe the range of seemingly anomalous effects which seem to arise from man’s interaction with the external universe. Such effects include clairvoyance (information about remote objects / events obtained by non- physical means); telepathy (communication between one mind and another without any form of physical transmission); precognition (predicting the outcome of future events to a greater degree of accuracy than would be expected given the currently available information) and psychokinesis (pk, mind having a direct and measurable influence on physical reality).
Clairvoyance, telepathy and precognition are often termed extra-sensory perception (ESP) and may be likened to a weak and unreliable sixth sense, e.g. a success rate of 27% opposed to a chance level of 25% over a large number of trials in a psi experiment would be considered a success. In contrast to ESP in which the subject appears receptive to non-physical information, pk involves the mind of the subject ‘reaching out” to influence the external world.
One of the strangest forms of psi is the phenomena known as retro-pk, in which subjects appear to be able to use pk to reach back in time and influence the outcome of already completed experiments.