Force, Inertia and the Constitution of Bodies: Leibniz's Reappropriation of Kepler in the 1690s. A New Account Based on an Unpublished Manuscript (LH 35, 15, 6, Bl. 28-29)

In many of his writings, Leibniz refers to Kepler’s concept of natural inertia to introduce his own idea of bodies having an intrinsic resistance to motion. This paper moves from the discussion of this concept of natural inertia as it can be found in a hitherto unpublished manuscript (ca. 1699-1700) in which Leibniz comments on Kepler’s Epitome Astronomiae Copernicanae. This manuscript shows the extent to which Leibniz sought to read Keplerian ideas through the lens of his dynamics. This text also contains one of the first references to ‘complex entelechy’, which will be crucial to Leibniz’s late account of substance. In the last section, we explore how Leibniz’s reinterpretation of Keplerian inertia must be read alongside his criticism of the Cartesian theory of matter in discussion with De Volder and Papin from the same years as our text. We also suggest that Leibniz proposed a sort of ideological reading of Kepler: by the end of the 1690s, he wanted to establish a German tradition in natural philosophy, especially in opposition to the English (i.e., Newtonian) one.