Regularly Irregular Motion in Proclus' Celestial Physics

As an accomplished mathematician and astronomer, Proclus was acutely aware that the increasingly complicated models employed by astronomers down to Ptolemy failed in their stated aim of reducing planetary orbits into regular circular motions. In Neoplatonic ontology, the celestial realm acts as intermediary between the intelligible and the sensible worlds. Observation of the heavens was thought to help the straying human soul recall divine order, and yet planetary motions also serve as proximate causes and paradigms of contrary motion in the sublunary world. In his attempt to reconcile the ideal of regular circular motion with the undeniable fact of observed irregularity, Proclus describes planetary motion in somewhat ambiguous terms: ‘regularly irregular’ or ‘irregularly regular’. Although a number of modern scholars have noted Proclus’ use of these terms, they have not taken it as an admission of genuine irregularity in the heavens. I contend that a thorough application of Proclus’ metaphysics to his theory of elemental motion, along with an appreciation of his modal approach to the philosophy of nature, allows for an interpretation in which this irregularity is real. As a result we are able to show conclusively that Proclus, while retaining the principle of uniform circular motion for higher ontological levels, is able to abandon this ideal with regard to the observable phenomena