Another Go-Around on Leibniz and Rotation

Abstract

his essay comments on the complexity of the task of accommodating Leibniz's account of relational motion with his dynamics, as evident in Anja Jauernig's (2008) Leibniz Review article, and suggests some possible strategies for overcoming these obstacles. There are few endeavors more problematic for the Leibnizian commentator than striving to shed some clarity on his various accounts of motion, a task which invariably ensnarls one in a thicket of associated, equally problematic issues, such as space, time, and force. In a recent essay, Anja Jauernig (2008) has made a concerted effort to untangle some of these difficulties. In what follows, I will lay out of the some of the problems that I believe reside in her approach, along with a few suggestions on a more adequate alternative. Not surprisingly, many of the difficulties pertain to rotation, which has long been recognized as the weak link in a relational theory of motion, i.e., the doctrine that all motion is the relative motion of bodies, with Leibniz's "Equivalence of Hypotheses" (EH) doctrine comprising an instance of a relational theory (or so it seems). The discussion of Leibniz's explanation of rotation, and why he thought it was still compatible with the EH (despite the claims of the Newtonians), will draw from his "Specimen Dynamicum" (AG, 117-137). In brief, Leibniz claims that the apparent centrifugal force manifested in a rotating body is not a problem for his theory of motion, since the individual particles that make up the rotating body do uphold the EH in their respective collisions with the surrounding plenum particles; hence the force effect is merely a result of these collisions, and, in fact, also explains the body's solidity, thereby undermining the claim that the centrifugal force effects of motion support absolute motions (in an absolute space). In her detailed investigation of the EH, which is informative in many ways, Jauernig explores the possibility of separating the phenomenal from the dynamical aspects of Leibniz's physical theory (where phenomenal pertains to the level of bodies, and dynamic to the minute corporeal substances that underlie phenomena; 12). The rationale behind this strategy is to establish that Leibniz's theory can employ both absolute and relational elements at these different levels, phenomenal and dynamic. Jauernig allows two possibilities (nicely summarized on 29-30), (1) that the structure of spacetime is Leibnizian at the phenomenal level and Galilean