 The idea that eternal truths exist necessarily in God, and do not depend on God’s causality was not new, and Mersenne here belonged to a rather well established scholastic tradition. An original feature of Mersenne’s position however is that he considered mathematical truths as the very paradigm of eternal truths. As pointed by Dear, this was not a received doctrine. Mathematics used to be considered not really worthy of the dignity of true sciences, on account of the inessential nature of their object and the non-causal character of their demonstrations. Mersenne, in the wake of Biancani (1615) defended the contrary view: mathematics are not only true sciences, but the highest among them, because of their certainty. Besides, their demonstrations are truly causal ones, as Mersenne showed with the example of numbers where unity appears to be the “material cause” and the intellect, conjoining and composing unities, the “efficient” one (cf. VS II: 284). Thus mathematics is able to describe the objects that God’s intellect considered when He created the world, that is the possibilia (Lenoble 1943)—or, better said, mathematics are the sciences of the metaphysically necessary attributes of all things.