The opinions of others, however authoritative, cannot produce the kind of certainty that reason alone can reach.

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