perception that occurs independently of the known sensory processes. Usually included in this category of phenomena are telepathy, or thought transference between persons; clairvoyance, or supernormal awareness of objects or events not necessarily known to others; and precognition, or knowledge of the future. Scientific investigation of these and similar phenomena dates from the late 19th century, with most supporting evidence coming from experiments involving card guessing. Subjects attempt to guess correctly the symbols of cards hidden from their view under controlled conditions; a better-than-chance percentage of correct calls on a statistically significant number of trials is considered to be evidence of ESP. Although many scientists continue to doubt the existence of ESP, people who claim this ability are sometimes used by investigative teams searching for missing persons or things.