Avoiding the Void: Avicenna on the Impossibility of Circular Motion in a Void*

The topic of the void was of significant philosophical and scientific importance in the ancient and medieval world. Some, such as the atomists, maintained that the void was essential if one were to explain motion. Others, such as Aristotle, argued that the existence of the void would absolutely preclude the possibility of motion. Moreover, there were disputes concerning even how to characterize the void. Thus the atomists claimed that interstitial voids were dispersed throughout every body and existed alongside bodies in an infinite space. Others, such as the Stoics, held that all bodies were localized in a plenum that was itself situated in an extra-cosmic, infinite void. Still others, such as the Neoplatonist John Philoponus, thought that the void was finite, and although as a matter of fact it is never devoid of a body, it at least is capable of existing independent of any body. The above roughly provides the gamut of positions concerning the void as it reached the medieval Arabic philosopher Avicenna ( 370–428/ 980–1037). In one form or another, Avicenna was aware of the various moves and counter-moves associated with the notion of the void. Like Aristotle, he maintained that the existence of the void would absolutely preclude the possibility of motion. His arguments in some cases simply rehearse those of Aristotle; in other places they expand on the thought of Aristotle in order to respond to new threats that arose after Aristotle’s own time; and in certain situations Avicenna constructs new arguments against the void used neither by Aristotle nor, from what we can gather, Aristotle’s later Greek commentators. In what follows I do not pretend to present Avicenna’s whole argument against the void; rather, I concentrate on one small subset within the broader complex of arguments associated with Avicenna’s refutation of the void. This is his argument that circular motion would be impossible within an infinite, void space. This argument is significant in that it shows a marked departure from Aristotle and what we know of other Greek Aristotelians; for neither Aristotle nor apparently his Greek commentators ever specifically treat the impossibility of circular motion in a void. Their arguments instead focus on natural and violent motion, where it is argued that violent motion, or motion opposed to nature, presupposes natural motion.They in turn show that natural motion in a void is impossible, concentrating onnaturalmotion in the category of place. This study falls into two sections: one, Avicenna’s argument and an analysis and assessment of it; and two, the historical and philosophical considerations motivating