Dr. Karl E. Zener (1903-1963) was a partner of Dr. Joseph B. Rhine (1896-1980), the parapsychologist who coined the term extrasensory perception (ESP). ESP refers to such alleged abilities as telepathy (mind reading), clairvoyance ("clear seeing"), clairaudience (extrasensory hearing), precognition (knowledge of future events), and psychokinesis (mind over matter).
To test for telepathy and clairvoyance, in 1930 Dr. Zener devised a set of cards imprinted with five symbols— circle, square, star, plus sign, and wavy lines— each repeated five times for a total of twenty-five cards. In some tests the subject tried to guess the order of a shuffled deck. Other trials involved a "sender" looking at each card in turn and the "receiver" attempting to telepathically divine the symbol.
Statisticians faulted the methodology employed, complaining for example of "optional stopping": arbitrarily ending a test when the subject was ahead, instead of determining in advance the number of cards to be guessed. Magicians complained of lax safeguards against cheating.
Zener cards (or the French Carte Zener shown above) are today usually called "ESP Cards" like the others shown. They are still used by parapsychologists who test for the elusive phenomenon. To date, ESP has not been established scientifically, despite frequent claims otherwise.