PierreDeFermat is often called the founder of the modern theory of numbers. Together with ReneDescartes?, PierreDeFermat was one of the two leading mathematicians of the first half of the 17th century. Independently of ReneDescartes?, PierreDeFermat discovered the fundamental principle of analytic geometry. His methods for finding tangents to curves and their maximum and minimum points led him to be regarded as the inventor of the differential calculus. Through his correspondence with BlaisePascal? he was a co-founder of the theory of probability.