Platonism and Planetary Motion: Reason, Balance and Order in Proclus’ Commentary on Republic 617a4–b4

Abstract

Abstract This paper focuses primarily on an examination of the discussion about planetary motion found in the portion of Proclus’ commentary on Plato’s Republic that deals with the myth of Er, commenting on Republic 617a4-b4. The passage is important because it provides evidence indicating that the Platonist Proclus argued for rejecting the hypothesis that planetary motion should be understood as some combination of circular movements. Some discussion of the Hypotyposis and the commentary on the Timaeus provides the background for the analysis of the passage in the Republic commentary. I propose that the portion of the commentary on the Republic that deals with the myth of Er is the latest of these three works and therefore develops more fully the ideas found in the Timaeus commentary and the Hypotyposis. The seriousness of Proclus’ long discussion of this short passage of Plato’s myth provides a new perspective for considering the ancient importance of Platonic myth in providing a role for interpretation and a framework for allowing the development of more complex scientific thought as part of the interpretative project. The paper particularly focuses on several key methodological statements which indicate a productive relationship between philosophy and the exact sciences and show serious attention to the problems of understanding the natural world by means of modeling as well as commitment to the importance of subjecting the fundamental premises of any system for understanding the natural world to continual rational examination. The paper also presents evidence indicating that Proclus made a new proposal for understanding planetary motion. This new proposal included the suggestion that mixed lines such as ellipses would provide a better solution than combinations of circles, and the paper argues that Proclus connects this new proposal to the idea that the heavenly bodies display in their movement some of the same causes of straight movement as earthly bodies. The goal of the paper is to provide a case study indicating how later Platonists understood Platonic theory in a way that included the importance of both textual authority and scientific progress and encouraged continual re-evaluation of leading scientific theories.