Extrasensory perception (ESP), also called a sixth sense or cryptaesthesia, is a claimed ability to receive information without using the usual physical senses, such as sight, hearing, or touch. Instead, this information is said to be sensed by the mind. The term ESP was used by J. B. Rhine, a botanist from Duke University, to describe psychic abilities like telepathy (reading minds), psychometry (gaining information from objects), and clairvoyance (seeing things not visible to others). These abilities may also involve seeing events before they happen (precognition) or after they occur (retrocognition).

Second sight is a claimed type of ESP in which a person sees visions about future events (precognition) or about things or events in faraway places (remote viewing). There is no strong proof that second sight exists. Reports about it are only based on personal stories. Both second sight and ESP are considered pseudosciences, meaning they are not supported by scientific evidence.

History

In the 1930s, at Duke University in North Carolina, J. B. Rhine and his wife, Louisa E. Rhine, studied extrasensory perception (ESP). Louisa focused on collecting stories about unexplained events, while J. B. Rhine worked in the laboratory. He defined terms like ESP and psi (parapsychology) and designed experiments to test these ideas. A set of black and white cards, originally called Zener cards and now called ESP cards, was created. These cards have five symbols: circle, square, wavy lines, cross, and star. Each symbol appears five times in a pack of 25 cards.

In a telepathy experiment, one person (the "sender") looks at the cards while another person (the "receiver") guesses the symbols. To test clairvoyance, the cards are hidden from both people while the receiver guesses. To test precognition, the order of the cards is decided after the guesses are made. Later, Rhine used dice to study psychokinesis.

The experiments at Duke University faced criticism from other scientists and scholars who questioned the evidence for ESP. Several psychology departments tried, but failed, to repeat Rhine's results. In 1936, W. S. Cox from Princeton University tested 132 people in a card experiment with 25,064 trials. Cox concluded there was no evidence of ESP in the people tested. He suggested differences in the experiments or the subjects might explain the results. Other psychology departments also failed to reproduce Rhine's findings.

In 1938, psychologist Joseph Jastrow said much of the evidence for ESP collected by Rhine and others was based on stories, not solid proof, and possibly unfair or unreliable. Rhine's experiments were later discredited because researchers found that mistakes or cheating could explain his results. For example, subjects might have seen the symbols from the back of the cards or noticed small clues from the experimenter.

In the 1960s, parapsychologists began studying the mental processes involved in ESP, the personal experiences of people who claim to have ESP, and how ESP might relate to psychology. This required new methods beyond Rhine's original approach. New experiments included studies on dream telepathy and the ganzfeld experiment, which involves reducing sensory input to test ESP.

The term "second sight" may have originated because normal vision was considered primary, while the ability to see beyond normal limits was seen as secondary. In Gaelic, "an dà shealladh" means "the two sights" and refers to the ability to see the future or distant events. This term is widely recognized, even though it technically means "two sights" rather than "second sight."

Skepticism

Parapsychology is the study of events that appear to go against normal rules, such as extrasensory perception (ESP). Over 100 years of research has not produced strong proof that ESP exists, leading many scientists to criticize the field. The scientific community does not accept ESP as real because there is no clear evidence, no explanation for how it might work, and no reliable experiments showing it is true. Scientists classify ESP as pseudoscience.

The scientific community does not consider extrasensory perception a real scientific phenomenon. Skeptics argue that no theory explains how ESP works, and past studies have shown problems with how experiments were designed.

Many experiments on ESP have methodological flaws, which weaken their results. These issues are not limited to one type of study and affect many experiments. For example, the Zener cards and Ganzfeld experiments share similar problems. One issue is the "stacking effect," where repeated use of the same card in a sequence makes it easier to guess without ESP. Another is "sensory leakage," where subjects see clues accidentally, like reflections on a blindfold. Poor shuffling of cards or marked cards can also make experiments unfair. A review of many studies found that even after fixing these errors, no strong evidence for ESP remained. Some studies seemed to show ESP, but this was likely due to the flaws in their methods.

In the early 1900s, Joaquin María Argamasilla, called the "Spaniard with X-ray Eyes," claimed he could read hidden messages through closed metal boxes. He tricked scientists like Gustav Geley and Charles Richet into believing he had psychic powers. In 1924, Harry Houdini exposed him as a deceiver. Argamasilla peeked through his blindfold and lifted the edge of the box to see inside without being noticed.

Science writer Martin Gardner noted that many people who investigate psychic abilities are unaware of how to trick others using blindfolds. He described how psychics like Rosa Kuleshova, Lina Anderson, and Nina Kulagina used techniques to peek from their blindfolds and mislead investigators into thinking they had special abilities.