Gottfried Wilhelm Leibniz (1646–1716) is best known among non-specialists for his development of the calculus and his incredible thesis that this is the best of all possible worlds. He was born in Hanover two years before the end of the horrific Thirty Years' War. After receiving a doctorate in law, he came to the notice of an influential converted Catholic, through whom he entered the service of the Elector of Mainz, another converted Catholic. Both encouraged one of Leibniz's lifetime passions: working toward Church reunion between Protestants and Catholics and ultimately between the various Protestant dominations. Leibniz's ecumenical work was characterized by good will, tolerance and flexibility. It says something about the cast of Leibniz's mind that he believed that if the differing sides could just agree on a correct metaphysics – his, as it turns out – reconciliation was possible.

In 1672 Leibniz was sent to Paris to present a plan devised to persuade the Sun King that his military ambitions would be manifested more gloriously if directed at Egypt rather than Germany. By the time Leibniz arrived, the political and military situation in Europe had changed enough that the plan was no longer relevant, but the four years Leibniz spent in Paris seem to have been the happiest of his life. There he met some of the most influential thinkers of the day, including Antoine Arnauld, Nicolas Malebranche and the mathematicians Walter Tschirnhaus and Christian Huygens, the latter of whom brought Leibniz up to speed on contemporary mathematical developments.