Gottfried Leibniz
The German Leibniz, already a member of the Royal Society, visited Leeuwenhoek in 1676. They corresponded until Leibniz's death in 1716.
Like many "natural philosophers" of the time, most notably Descartes and Spinoza, Leibniz was a rationalist, seeing a world of simple, interconnected rules. He was also a polymath, best remembered for having developed the calculus about the same time as Newton. He also developed the binary counting system that is used in digital computers today. His only book-length work is Theodicy, a work of metaphysics. In it, he used Leeuwenhoek to support his theory.
It is thus my belief that those souls which one day shall be human souls, like those of other species, have been in the seed, and in the progenitors as far back as Adam, and have consequently existed since the beginning of things, always in a kind of organic body. On this point it seems that M. Swammerdam, Father Malebranche, Mr. Bayle, Mr. Pitcairne, M. Hartsoeker and numerous other very able persons share my opinion. This doctrine is also sufficiently confirmed by the microscopic observations of M. Leeuwenhoek and other good observers.
From 1691 to 1716, Leibniz was the librarian of the Herzog August Bibliothek in Wolfenbüttel. Its patron was Anton Ulrich, grandson of the library's founder, who visited Leeuwenhoek in either 1691 or 1709, the two times he traveled to the Dutch Republic.
In Leibniz's correspondence, he notes that Leeuwenhoek sent his letters to Germany via Joachim Georg Reinerding, the agent in the Dutch Republic of the Hof von Wolfenbüttel. He was the son of Johann Thiele Reinerding (-1727), after 1684 the first secretary of the Herzog August Bibliothek.
Leibniz wrote his Monadology after reading Leeuwenhoek's letters in Philosophical Transactions.
Now nothing better corroborates the incomparable wisdom of God than the structure of the works of nature, particularly the structure which appears when we study them more closely with a microscope. . . . A man in Delft [van Leeuwenhoek] has accomplished wonders at it, and if there were many others like him, our knowledge of physics would be advanced far beyond its present state.
Reflections on the Common Concept of Justice, 1702
On March 2, 1691, he wrote to Christiaan Huygens:
I am more fond of a Leeuwenhoek who tells me what he observes, than of a Cartesian who tells me what he thinks. Nevertheless, it is inevitable to join reasoning with observations.
He also communicated about Leeuwenhoek with Louis Bourguet and Hendrik van Bleiswijk.
