This letter of August, 1641, can be found in Correspondance du P. Marin Mersenne, religieux minime. X, Du 6 août 1640 à fin décembre 1641 (1967: 730) wherein the author praises Sanches’ ingenious exaggerations concerning the possibility of acquiring perfect knowledge, and criticizes Cartesianism for not being able to advance (“non progreditur”) beyond its “hyperbolical doubt” (1967: 730) . But again, the letter is too late to serve as proof of Descartes’ knowledge of Sanches’ treatise prior to t