The following year he met the diplomat Baron von Boineburg, at whose suggestion he entered the diplomatic service of the Elector of Mainz.The years 1672 to 1676 he spent as diplomatic representative of Mainz at the Court of Louis XIV.As a scientist he appreciated and encharged the use of observation and experiment: "I prefer," he said, "a Leeuwenhoek who tells me what he sees to a Cartesian who tells me what he thinks." As a historian he emphasized the importance of the study of documents and archives.
At Paris he had come to know many prominent Jesuits and Oratorians, and now he began his celebrated correspondence with Bossuet.
During this time he paid a visit to London and made the acquaintance of the most learned English mathematicians, scientists, and theologians of the day.
While at Paris he became acquainted with prominent representatives of Catholicism, and began to interest himself in the questions which were in dispute between Catholics and Protestants.
When Leibniz became librarian and archivist of the House of Brunswick in 1676, the Duke of Brunswick was Johann Friedrich, a recent convert to Catholicism.
Almost immediately Leibniz began to exert himself in the cause of reconciliation between Catholics and Protestants.