Like most seventeenth-century intellectuals and scholars, Descartes entertained a correspondence, letters being in this period the main channel for the communication of scientific news. Letters were often shared with others, and this was one of the reasons why their authors usually kept an archive of minutes, first drafts, or full copies. In Paris and other centers of learning, there were mostly informal circles of friends (often called “academies”), in which letters were read and discussed on a weekly or even a daily basis. Usually they constitute an essential part of a philosopher's or scientist's legacy, not only because they illuminate the various stages through which an idea passed before being known to the public, but also because they give insight into, for example, secret or immature thoughts or discarded plans. All of this can also be said of the correspondence of Descartes, whose letters serve by and large the same purpose as those of other philosophers and scientists of the period – to react to news and gossip, to discuss plans, to propose and solve problems, to deal with objections, and to discuss and communicate data. However, in some respects his correspondence is peculiar. First of all, by its quantity: the volume as we know it (and much of it was lost) is almost twice that of his published works and four times that of his posthumous works. Moreover, although Descartes was in contact with many of his contemporaries, few people wrote to him directly: most of the French correspondence passed through the hands of Mersenne. Finally, the way in which Descartes’ correspondence survives poses an enormous challenge.

Before leaving the Low Countries in the autumn of 1649, Descartes entrusted Cornelis Van Hogelande with a trunk containing letters and papers, instructing him that these should be burned in the event of his death. Presumably, these were mainly letters written to Descartes since he took the minutes of his correspondence and his unpublished manuscripts with him to Sweden. Whether Van Hogelande did burn the letters we do not know. In one case (that of Constantijn Huygens), it is certain that he restituted the letters to their sender. After Descartes’ death, an inventory was made of his possessions. Chanut, the French ambassador in whose house Descartes died, sent personal belongings to the Descartes family but kept his scientific writings, including the general correspondence.