 By the end of the 1620s, Mersenne’s former animosity towards the anti-aristotelians authors withered, giving way to a more lenient if not tolerant approach. Galileo whom he had listed among the dangerous ones in 1623 is now the object of great praise. Mersenne seemed much less concerned with the proliferation of atheists and heretics, and he expressed regrets about the offensive tone of his previous works (Letter to Rivet 12 nov. 1639, MC X: 599). His travel to Holland in 1630 put him in contact with Protestant scholars, such as André Rivet and the Socinian Ruarus, who became regular correspondents. Mersenne who used to vilify the heretics, was now taken to task for his sympathies with the enemy, and he was sometimes dubbed the “Huguenot monk”. In any case the staunch defense of orthodoxy of the first works is nowhere to be found in Mersenne’s next publications, which, from 1634 onward, are almost entirely devoted to the promotion of the mathematical sciences, broadly conceived. It has been a question among biographers and commentators whether these changes reflect a conversion to a new agenda or a new philosophical outlook. Robert Lenoble (1943: 38) considers that until 1634, Mersenne was still looking for a better philosophy of nature and held provisionally to Aristotelianism, “whereas at that date he discovered what he had sought for many years, namely Mechanism”. Sylvie Taussig (2009b) considers that Mersenne went through a serious crisis, both moral and intellectual, at the end of the 1620s, and that under the influence of Peiresc and Gassendi he came to adopt a more skeptical outlook on philosophical and religious matters. Without denying Mersenne’s change of tone and accent, Garber (2004) and Dear (1988) tend to underplay the extent and meaning of Mersenne’s “conversion” to the new philosophy, considering that his agenda of promoting mixed mathematics was to him still compatible with a traditional philosophical outlook—“the learning of the schools”, that Dear described as conjoining Christianized Aristotelianism, Renaissance humanism and a skeptical or probabilistic approach to philosophical matters. We shall come back to these issues in due course.