Most studies of René Descartes’s legacy have focused on the novelty of his ideas, but little has been done to uncover the conditions that allowed these ideas to spread. Seventeenth-century Europe was already a small world—it presented a high degree of connectedness with a few brokers bridging otherwise disparate regions. A communication network known as the Republic of Letters enabled scholars to trade ideas—including Descartes’s—by means of correspondence. This article offers an analysis—both qualitative and quantitative—of a corpus of letters written (...) during Descartes’s lifetime and mentioning his name. The aim is to unveil the factors that drove the diffusion of Descartes’s ideas. The results are twofold. First, a close reading of the letters reveals that they were not used to create awareness about Descartes and his works but rather to discuss his ideas. Second, a network analysis of the letters shows that ideas do not spread like viruses, thus undermining the ‘social contagion’ model, and that weak ties are not as effective in promoting innovations as they are in circulating information. (shrink)