Leibniz was perhaps the last great universalist, contributing to disciplines from biology to probability and from theology to linguistics. He was one of the three great 17thC rationalist philosophers, the inventor (independently of Newton) of infinitesimal calculus, and advanced the design of calculating machines. Huygens was a mentor. Leibniz spent several days in deep discussion with Spinoza, met Boyle, Leeuwenhoek and Goldbach, and corresponded extensively with the Bernoullis, von Tschirnhaus, Arnauld, Bayle, Sloane and Oldenburg. Newton and he never met nor corresponded directly. Wallis wrote refusing him permission to teach cryptography to students in Hannover.