Pierre de Fermat (1601-1665), French mathematician and physical theorist. Fermat did not publish his work in the usual sense and his results (often without proofs) are known through his letters to friends. He corresponded on probability theory with Pascal. Fermat also worked on number theory, analytical geometry and the maximum and minimum values of functions. The proofs for Fermat's work were largely published by Euler, but Fermat's Last Theorem was not proved until 1993 (by Andrew Wiles). In physics, Fermat devised Fermat's principle to derive the path taken by light rays, and used it to deduce the laws of reflection and refraction of light.