 The conjoined effects of Mersenne’s epistemological pessimism and metaphysical voluntarism make his version of “mechanism” very different from the Cartesian, Hobbesian or Gassendist one. He never conceived mechanical explanations as a new science of “causes”, based on the adoption of new principles and elements intended to take the place of Aristotelian substantial forms—such as Descartes’s subtle matter or Gassendi’s atoms. Mechanics, as all other mixed-mathematics, was simply concerned with establishing the laws of phenomena.