Extrasensory perception

Extrasensory perception (ESP), sometimes called a sixth sense or cryptaesthesia, is the supposed paranormal ability to pick up information through the mind rather than through the usual physical senses. The term was coined by J. B. Rhine, a botanist at Duke University, to cover psychic abilities like telepathy, psychometry, clairvoyance, and their time-related forms—precognition (seeing the future) and retrocognition (seeing the past). A related idea is second sight, where a person gets a vision of future events or distant places. However, there is no solid evidence that second sight is real; reports come only from personal stories. Both second sight and ESP are considered pseudosciences.

In the 1930s, Rhine and his wife Louisa E. Rhine conducted research at Duke University. Louisa gathered accounts of spontaneous experiences, while J. B. focused on lab work, defining terms like ESP and psi, and designing experiments. They created a simple set of cards, originally called Zener cards and now known as ESP cards, with five symbols: circle, square, wavy lines, cross, and star. A pack has 25 cards, five of each symbol. In a telepathy test, a "sender" looks at the cards while a "receiver" guesses the symbols. For clairvoyance, the pack is hidden from everyone while the receiver guesses. For precognition, the card order is set after the guesses are made. Rhine later used dice to test psychokinesis.

These experiments drew criticism from academics and others who doubted the evidence. Several psychology departments tried to repeat Rhine’s work but failed. For instance, W. S. Cox at Princeton University ran 25,064 trials with 132 subjects using playing cards and found no sign of ESP. He said the difference between his results and Rhine’s likely came from uncontrolled factors or differences in the subjects. Four other departments also failed to replicate the findings. In 1938, psychologist Joseph Jastrow argued that much of the evidence was anecdotal, biased, and due to faulty observation. Rhine’s results were later discredited because sensory leakage or cheating—like subjects seeing card symbols from the back or picking up subtle clues from the experimenter—could explain them.

In the 1960s, parapsychologists shifted focus to the cognitive side of ESP, including the subjective experience of making ESP responses and its role in psychological life. This led to new methods beyond Rhine’s forced-choice approach, such as dream telepathy experiments and the ganzfeld procedure, which uses mild sensory deprivation.

The term "second sight" may have come from the idea that normal vision comes first, while supernormal vision is secondary and limited to certain people. In Gaelic, it’s called *an dà shealladh*, or "the two sights," meaning a seer’s ability to see future or distant events. Many Gaelic words describe different aspects of second sight, but *an dà shealladh* is the most familiar to non-Gaelic speakers, even though it literally means "two sights," not "second sight."

Parapsychology, the study of ESP and other psychic phenomena, has been criticized for continuing research despite failing to provide convincing evidence after over a century. The scientific community rejects ESP because there is no evidence base, no theory to explain it, and no positive experimental results. Skeptics note that no viable mechanism exists for ESP, and many studies have had flaws in their design. These flaws aren’t limited to one type of experiment. For example, the Zener card tests and the ganzfeld experiments both suffer from the "stacking effect," where trial-by-trial feedback in a closed sequence (like a card deck) violates the statistical assumption of independence. This makes card counting easier and raises the chance of correct guesses without ESP. Another problem is sensory leakage, such as a subject seeing a card’s reflection in the experimenter’s glasses.