 Although these features of Mersenne’s activities (as an animator of scientific life and as Descartes’s friend and correspondent) are the most well-known, they should not conceal Mersenne’s own original contribution to philosophy. While his first works were polemic in tone, offering “scientific apologetics” (Lenoble 1943) using arguments taken from the sciences to defend Christian catholic religion against all sorts of heterodox trends threatening it, his latter publications (from 1634 onward), far less concerned with apologetics, illustrated and promoted the new mechanical, mathematical, and experimental sciences. A key thread running through his complex intellectual career, music, understood as the general science of harmonics, received Mersenne’s constant attention and was the subject of several important publications.