Extra-sensory perception, or ESP, is the ability to acquire information by means other than the known senses, i.e. taste, sight, touch, smell, hearing, balance and proprioception. The term implies sensual sources of information unknown to science. The existence of ESP has not been scientifically established and is uniformly discredited by the scientific community.

Specific types of extra-sensory perception include:
Perception of events in other places (clairvoyance, clairaudience, clairgustance, clairsentience) and in other times (precognition, retrocognition, second sight)
Perception of aspects of others not perceivable by most people (aura reading)
The ability to sense communications from, and communicate with, people far away (telepathy), beyond the grave (medium-hood and séancing, spirit walking), or in other dimensions (astral projection)

The study of these abilities, called parapsychology, includes other phenomena such as psychometry and psychokinesis, which are associated with ESP.

A person capable of using ESP is refered to as a psychic or as having psychic powers.

The notion of extra-sensory perception existed in antiquity. In many ancient cultures, such powers were ascribed to people who purported to use them for second sight or communicate with deities, ancestors, spirits, etc.

Extra-sensory perception and hypnosis

When Franz Anton Mesmer and Grigori Rasputin were first popularizing hypnosis, the legend came about that a person who was hypnotized would be able to demonstrate ESP. Carl Sargent, a psychology major at the University of Cambridge, heard about the early claims of a hypnosis-ESP link, and designed an experiment to test whether they had merit. He recruited forty fellow college students, none of whom identified him- or herself as having ESP, and then divided them into a group that would be hypnotized before being tested with a pack of 25 Zener cards, and a control group that would be tested with the same Zener cards. The control subjects averaged a score of 5 out of 25 right, exactly what chance would indicate. The subjects who were hypnotized did more than twice as well, averaging a score of 11.9 out of 25 right. Sargent's own interpretation of the experiment is that ESP is associated with a relaxed state of mind and a freer, more atavistic level of consciousness.

This experiment was later shown to have been conducted without normal experimental controls. Subsquent experiments using normal experimental controls have not reproduced Sargent's results or any indication of the existence of ESP.

In the 1930s, at Duke University in North Carolina, J. B. Rhine and his wife Louisa tried to transform psychical research into an experimental science. To avoid the connotations of hauntings and the seance room, they renamed it “parapsychology”. While Louisa Rhine concentrated on collecting accounts of spontaneous cases, J. B. Rhine worked largely in the laboratory, carefully defining terms such as ESP and psi, and designing experiments to test them. A simple set of cards was developed, originally called Zener cards (after their designer)—but now called ESP cards. They bear the symbols circle, square, wavy lines, cross, and star; there are five cards of each in a pack of 25.

In a telepathy experiment the “sender” looks at a series of cards while the “receiver” guesses the symbols. To try to observe clairvoyance, the pack of cards is hidden from everyone while the receiver guesses. To try to observe precognition, the order of the cards is determined after the guesses are made.

In all such experiments the order of the cards must be random so that hits are not obtained through systematic biases or prior knowledge. At first the cards were shuffled by hand, then by machine. Later, random number tables were used and nowadays, computers. An advantage of ESP cards is that statistics can easily be applied to determine whether the number of hits obtained is higher than would be expected by chance. Rhine used ordinary people as subjects and claimed that, on average, they did significantly better than chance expectation. Later he used dice to test for PK and also claimed results that were better than chance.

Rhine's controversial book, Extrasensory Perception After Sixty Years (1940), led others to criticize his methods and to try to repeat his findings. Most failed, including the London mathematician Samuel Soal, who tried for five years without success. Eventually he re-analysed many of his results and found that one subject was apparently performing precognition. In the early 1950s, further tests with this subject, under tightly controlled conditions, gave statistically significant results—convincing many people that Rhine was right. Accusations and counter-claims abounded until, in 1978, it was finally proven that Soal had cheated and the results were worthless. However, many people had been convinced by these results for nearly 30 years.

Other parapsychologists found that some subjects scored below chance (psi-missing); scores tended to decline during testing (the “decline effect”); and people who believed in psi, called “sheep”, scored better than those who did not believe in it (“goats”)—which became known as the sheep-goat effect. However, none of these effects proved easy to replicate. In recent years parapsychologists have turned to other methods, notably free-response ESP tests and micro-PK.

Proponents of the existence of ESP point to numerous scientific studies that appear to offer evidence of the phenomenon's existence: the work of J. B. Rhine, Russell Targ, Harold E. Puthoff and physicists at SRI International in the 1970s, are often cited in arguments that ESP exists.

Numerous ESP studies have failed to find any evidence of the phenomenon. Those studies that have produced evidence of its existence are marred by methodological flaws . Proponents of ESP claim that the phenomena is a "taboo" subject in the scientific and rationalist communities, resulting sociological rather than scientific barriers to research.