 Epistemological, metaphysical and aesthetic aspects are intertwined in Mersenne’s musical agenda. Music was to Mersenne a mixed-mathematical science, just as astronomy, whose primary aim was to “save the phenomena” through more or less plausible hypotheses. The musical phenomena par excellence is “consonance”, that is the fact that certain sounds put together are agreeable to the ear. Recent musical practices, new instruments and polyphonic music had disclosed new modes of consonance that were previously unknown, and Mersenne saw his proper task in an effort to find the best physico-mathematical hypotheses that would be able to account for them, and could be used to invent more “perfect” musical pieces (chants). Seeking such hypotheses, Mersenne relentlessly asked his correspondents how they could account for the fact that certain sounds put together are agreeable and others not. For example, in November 1627, he asked a correspondent in Rome: