Leibniz, who was born in Leipzig and died in Hanover, was one of the greatest intellects of all time. He made significant contributions to a bewildering number of fields of endeavor. Perhaps his most lasting achievement was his discovery of the differential calculus. However, Leibniz also did other pioneering work in mathematics, including the invention of the binary number system, and he wrote works in disparate areas of natural philosophy, from physics and chemistry to geology and botany. He was employed for almost forty years by the court of Hanover as a counselor and charged, among other things to write on a history of the House of Brunswick. In connection with this lengthy project, he collected many documents relating to the history of Europe and the origins of France and Germany. Leibniz also took a serious interest in the Jesuit mission to China, which led to writings on Chinese history and philosophy. He also actively engaged in diplomatic work for much of his career and, as well as producing an extensive correspondence related to issues of state, was a tireless champion of the cause of the reunification of Catholic and Protestant churches. This more practical side of Leibniz's character extended to technological innovation. He invented one of the first calculating machines and spent several years in the mining area of Harz attempting to design and build windmills that served to drain away water.

As a philosopher, Leibniz is perhaps best known for his claim that the actual world is the best of all possible worlds, as lampooned mercilessly in Voltaire's Candide. However, his longevity as a subject of academic study is driven largely by his commitment to the claim that the created world is ultimately composed of an infinite number of unextended, mind-like individuals, which he called “monads.”

Descartes and Cartesianism figure prominently in Leibniz's writings, and it is arguable that much of his philosophy and natural philosophy emerged as a direct response to the Cartesian tradition, which was already becoming dominant by the time he reached his maturity. References to Descartes can be found among the earliest of Leibniz's writings, which date from the late 1660s and early 1670s.

Leibniz never wrote a magnum opus. His books were generally limited in scope (e.g., the Dynamics and Theodicy), and the one exception (the New Essays) is shaped as much by Leibniz's desire to show the inadequacies in Locke's views as by his desire to provide an exposition of his own philosophy. Where Leibniz did present surveys of his philosophy, these took the form of essays which were published in the journals of the day or which lay hidden from public view until after his death. And these essays, even the most famous ones such as the Discourse on Metaphysics, the New System, and the Monadology, provide little more than brief, and often exasperating, sketches. Nowhere do we find the likes of Descartes's Meditations, Spinoza's Ethics, or Locke's Essay.

The task of understanding Leibniz's philosophy would be a difficult one, to say the least, were it not for an additional fact. For Leibniz was unlike his contemporaries in another way, namely in the extent to which he was willing to engage in philosophical discussion through the medium of correspondence. To be sure, students of other major figures have letters available to them. For example, eighty-four letters between Spinoza and his correspondents have survived – although almost half of these were written to Spinoza rather than by him. And Descartes left a larger body of correspondence consisting of just over six hundred letters, around five hundred of which were written by Descartes himself.

Much of Leibniz's philosophy can be seen as a direct response to the views of Descartes. Sadly, there was no interaction between these two giants. Leibniz was only three years old when Descartes died, and even he had not begun to court the favor of luminaries from the Republic of Letters at this tender age. However, two of the most important of Leibniz's correspondences, with Antoine Arnauld and with Burchard de Volder, allow us to gain some insight into how he might have interacted with the master. The latter correspondence will be my concern here.

Scholars have long recognized the importance of Leibniz's correspondence with De Volder and have drawn frequently on passages from Leibniz's letters, both in book-length studies and in recent debates over the precise nature of his mature ontology. However, De Volder's contribution to the correspondence has received little attention, despite the fact that an understanding of the views which both correspondents bring to the discussion is essential for a proper interpretation of Leibniz's pronouncements.

The main role which De Volder plays in his correspondence with Leibniz is that of apologist for a particular brand of Cartesianism. De Volder's abilities as a disputant, and his genuine perplexity concerning the nature of the material world, force Leibniz to illuminate his own views in opposition to Cartesian doctrines on substance, matter, and motion.