Psi ψ

Psi is the twenty-third Greek letter, a Greek innovation without a Phoenician model; its value is 700. Its trident-like shape has been adopted in modern times as the emblem of psychology and psychiatry (for psyche); it has no significant ancient mystical tradition beyond its place in isopsephy. The Greek letters doubled as numerals in the Milesian (alphabetic) system, in which each letter carried a fixed value; the practice of summing a word's letters into a number, called isopsephy, is the Greek counterpart of Hebrew gematria and appears in inscriptions, the Greek Magical Papyri and early Christian texts.

Psi is one of four supplementary letters, together with Phi, Chi and Omega, added at the end of the Greek alphabet once its Phoenician-derived series had run out; like Phi and Chi before it, it has no Semitic ancestor and no inherited name of its own, its name likewise built simply from its sound plus a vowel, in the pattern of Greek's other late, home-grown letters. Its isopsephy value of 700 belongs to the Milesian alphabetic numeral system, which also needed three further, otherwise obsolete letters, digamma (also called stigma), koppa and sampi, to cover every value from 1 up to 900; none of the three survives among the 24 letters in ordinary use today.

Psi's two most prominent modern uses sit at opposite ends of respectability within the sciences. In quantum mechanics, physicists conventionally use an uppercase Psi to denote the wave function in the Schrodinger equation, formulated by Erwin Schrodinger in 1925 and 1926, a mathematical object that encodes the probability of finding a particle in a given state and remains one of the central objects of modern physics. In parapsychology, by contrast, researchers from the mid-20th century onward adopted lowercase psi as an umbrella term for claimed psychic phenomena such as telepathy and precognition, a use popularised by the parapsychologist Robert Thouless in 1942 specifically because the letter carried no prior connotation that might bias interpretation of the phenomena under study; mainstream science has never validated the phenomena the term covers, and the psi research programme remains outside the scientific consensus.

Astronomers, following the Bayer star-naming convention of 1603, also use Psi for one of the fainter numbered stars in a constellation, a third, much quieter modern role with no connection to either of the other two. In the Milesian numeral system, discussed elsewhere in this atlas, psi carries the value seven hundred, a value with no bearing on quantum mechanics, parapsychology or star catalogues alike.