The term 'psi' was introduced to be a neutral term for a variety of 'paranormal' phenomena, free of the implicit assumptions that are contained within labels such as psychokinesis ('movement by mind') or telepathy ('feeling or perception at a distance'), and of the whole assumption of paranormality. It was originally split into two categories of psi-kappa and psi-gamma, denoting active and passive psi, the first subsuming those phenomena such as psychokinesis which appeared to involve an effect upon another system, the second covering phenomena such as extrasensory perception, which showed a more receptive aspect. The main problem with the terms is that they do not have a precise definition, indicating which phenomena may be counted as psi and which are excluded. Psi has been defined as the 'unknown factor' in psychic experiences, and often this is modified to being a factor which is not reliant on the conventional sensorimotor channels.