 The bulk of Mersenne’s contribution to rational theology is contained in the 700 or so first columns of his Quaestiones in Genesim, where no less than 35 proofs of God’s existence are reckoned and detailed. The importance of rational demonstration is directly related to the apologetic project: one cannot simply be content with the idea that God is immediately known, as this could not shut the mouths of the atheists. The encyclopedic turn of the recension is obvious, and Mersenne, ready to use all means available to contradict the atheists, was not taken aback by the fact that these proofs belong to various, and sometimes adverse, theological traditions. After summoning the consensus of all creatures, and the consensus of all men, Mersenne resorted to cosmological arguments, arguing, from the contingency of our word, and the contingency of our own finite existence, that “it is necessary that something exists by itself”. Here Mersenne borrowed from Hugues of Saint Victor and Jean Damascene.