The aim of this paper is to give a concrete example of how network analysis can be applied to study the dissemination of new ideas throughout history. The father of modern philosophy, René Descartes (1596--1650), is taken as a case study. To trace the rapid spread of Descartes' ideas across early modern Europe, I will rely on ePistolarium, a web archive of more than 20,000 digitized letters. Using the metadata and transcriptions of ePistolarium, I will model a network in which the edges will represent the letters mentioning Descartes and the nodes will represent the people connected by those letters. Studying the topology of the network, I will be able to determine which actors were more involved in spreading Descartes' ideas and the routes along which information about Descartes travelled.