De motu: Leibniz on the Relativity of Motion Chapters

In this chapter it is shown how Leibniz distinguished between motion in its geometrical sense, where it is mere change of situation, and motion with respect to cause. Motion in the former sense is abstract and essentially relative, whereas the identification of which bodies are actually in motion is achieved by recourse to the “most intelligible hypothesis,” the one best explaining the origins of the motions under consideration. It is explained how on this basis Leibniz could distinguish true from merely apparent motions, even while rejecting the idea that true motions are motions with respect to an existing immobile space, and how he used this distinction from 1676 onwards to defend Copernicanism without this committing him to instrumentalism. The relationship of his views on the relativity of motion to his emerging dynamics in the 1680s and ’90s is examined, and the idea that vis viva is a criterion for determining which bodies are really moving is refuted. Commentators’ claims that derivative force acts on a different ontic level from primitive force, or that bodies are on a different ontic level from forces, are also examined and refuted. Leibniz’s views on the relativity of rotational motion are analysed and found wanting. The last section gives an extended analysis of the way in which Leibniz treated motion through space and time in his analysis situs, showing how the “spacetime” in question is a mental construction for determining motions on the basis of the most intelligible hypothesis, rather than a really existing thing.