The Kinematic Logic of Relational Transfer: An Unwritten Chapter in the History of Cartesian Motion

Abstract

While nearly every aspect of Descartes’ much-maligned theory of motion has been carefully scrutinized by commentators, there are a few hypotheses advanced in the Principles of Philosophy that have been, oddly enough, largely overlooked. Essentially, these criteria constitute an amendment to the Cartesian doctrine of relational motion: i.e., that motion is the reciprocal translation of a body and its contiguous neighborhood (of surrounding plenum bodies). In his neglected refinements to this view, Descartes argues that there is a way to determine which of the two reciprocally translating bodies is really in motion. When two smaller bodies move in opposite directions on the surface of a larger body, he explains, the kinematics of the scenario rule out the possibility that it is the larger body that is in motion. Moreover, a body is in motion if its whole surface, and not merely a portion of its surface, moves relative to its neighborhood. The importance of these “supplementary” criteria cannot be overestimated, for they would seem to compromise the “strong” form of relational motion normally attributed to Descartes; since, to be more specific, these criteria would appear to challenge Descartes’ principal judgment that motion is a purely reciprocal change of a body’s contiguous neighborhood.