Pierre de Fermat (1601-1665). Caricature of the French mathematician and physical theorist Pierre de Fermat. Fermat is most famous for devising Fermat's principle to derive the path taken by light rays, and using it to deduce the laws of reflection and refraction of light. He 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. Fermat's Last Theorem was not proven until 1993 (by Andrew Wiles).