 From the years 1620 onward, Mersenne deployed overflowing intellectual activity. Through the mediation of Claude Fabbri de Pereisc whom he had met in 1616, he was introduced to the Parisian intellectual elite, and later to Pierre Gassendi with whom he remained close friends. In Paris, he associated with the mathematician Claude Mydorge, and, perhaps through him, renewed acquaintance with Descartes. Mersenne called on Descartes’s mathematical expertise on questions of optics. His questions led Descartes to his first formulation of the law of refraction, which he was to publish later in his 1637 “Dioptrics”. Mersenne was among those attending the famous meeting of 1628, at the residence of the apostolic nuncio, where Descartes, having revealed his project of founding the sciences on a new method, had been encouraged to do so by the highest religious authorities. After Descartes left for Holland, Mersenne remained in close contact with him through correspondence. He was privy to most of Descartes’s secret addresses, which made him the man one has to see in order to get access to the French exile, to send him letters or questions. Only four letters from Mersenne to Descartes are extant, but 146 letters from Descartes—about a quarter of his known correspondence—testify to the intensity and continuity of the exchange. Mersenne endlessly provided Descartes with books, fresh information, and editorial services, requesting in exchange answers to queries of all sorts. Acting as mediator in Descartes’s controversies, with Fermat, Roberval, Beaugrand, Morin, or Voëtius, he sometimes caused quarrels himself, through his indiscretions, such as the one which made Descartes definitively fall out with his erstwhile mentor Isaac Beeckman. Mersenne also acted as intermediary for the edition of the Meditationes de prima philosophiae, and was instrumental in the gathering of the six sets of objections that were added to the text with Descartes’s responses, including those of Hobbes and Gassendi. He contributed himself to the objectiones, penning the second set (“gathered by the R. P. Mersenne from the mouth of various theologians and philosophers”, AT IX-1: 96; Buccolini 2019) and perhaps parts of the sixth.