 This may shed some light on Mersenne’s role in the writing of the second objections to Descartes’s Meditations, where doubts were cast on the demonstrative character of the Cartesian proofs of God’s existence, and especially on the ontological proof, whose Anselmian version was so much favoured by Mersenne in his younger years. It has been argued (Garber 2001), quite convincingly, that Jean-Baptiste Morin, a member of the Mersenne circle, and the author of a booklet on God’s existence written in a geometrical fashion (Morin 1635) had an important hand in the composition of the second objections and that he was probably the one who suggested the request of presenting the Meditations arguments in a geometrical manner. Mersenne, however would have concurred with the reservations, implicit in this demand, concerning Descartes’s claim to the higher certainty of metaphysical demonstrations over mathematical ones.