 It is to be noted that the two a priori arguments (from eternal truths and from the idea of a perfect being) are the only two that are brought in in the Impiété to dispel atheism. In the Quaestiones however more than half of Mersenne’s arguments were a posteriori ones: from the creation ex nihilo, from the beauty of creatures, from the given natural law, from the harmony of the universe… All allowed lengthy scientific excursuses, demonstrating God’s intelligence and providence through whatever the sciences can show of the order of things. Among these arguments, Mersenne paid only lip service to the classical Aristotelian and Thomist “argument from motion”, which states that the world needs an immutable first mover, probably because he is aware that the argument had been used in Paduan Aristotelianism in order to defend the coeternity of the world and its first mover. In the other a posteriori proofs, the regressive move from the creation to the creator was not as likely to subject the creator to the creation.