In mathematics and other scientific disciplines a common practice is to name a theory, an equation, and other discoveries in honor of the scientist who pioneered the investigation. Some examples of such expressions are Galois theory, Fahrenheit scale, Freudian psychoanalysis, pasteurization, Zorn's lemma, Planck's constant, Linnaean system of botanical classification, Hilbert space, Darwin's theory of evolution, Halley's comet, Keynesian economic theory, Mendelian genetics, and so on.