Pierre de Fermat (August 17, 1601 - January 12, 1665) was a French mathematician who is generally given minor credit for the development of modern calculus; in particular, for his work regarding tangents and stationary points. His work was such that he is sometimes regarded as the "father" of, both, differential calculus and number theory. He also made notable contributions to analytic geometry and probability.
Fermat worked on number theory while preparing an edition of Diophantus, and the notes and comments thereon contained the numerous theorems of considerable elegance necessary to develop the theory of numbers. Fermat is famous for his "Enigma" that was an extension of Pythagorean Theorem, also known as Fermat's Last Theorem. Together with René Descartes, Fermat was one of the two leading mathematicians of the first half of the 17th century. Independently of Descartes, he discovered the fundamental principle of analytic geometry. Through his correspondence with Blaise Pascal, he was a co-founder of the theory of probability.