 Mersenne made a considerable effort to do and redo experiments that his predecessors (most notably Galileo) had not always had the sense to present precisely. Thus, on the subject of the increase in speed during falling motion on an inclined plane, he conducted a precise campaign of measurements that enabled him to criticize Galileo’s numerical results, and eventually express doubts on the validity of the law itself (Palmerini 2010). As usual, he expressed his reservations concerning our capacity to determine with certainty the causes of gravity: “it is just as difficult to find the real cause as to demonstrate whether the earth is stable or mobile” (Traité des mouvements) and he placed back-to-back the reasons put forward by his contemporaries—positive and real quality, air pressure, magnetic attraction—which could only partially account for the phenomenon in its precise numerical dimensions. As Peter Dear has shown, Mersenne’s approach to the science of motion was inductive in nature, not centered, as it was in Galileo, on the abstract physical process, but rather on “a generalized description of cases, whose details and precise parameters held an importance altogether lacking in Galileo” (Dear 1994).