 But if the Earth does not offer a unique frame of reference for the description of motion, then we may wonder whether any arbitrary object can be used for the definition of motions: are all such motions on a par, none privileged over any other? It is unclear whether anyone has really consistently espoused this view: Aristotle, perhaps, in the Metaphysics. Descartes and Leibniz are often thought to have done so; however, as we’ll see, those claims are suspect. Possibly Huygens, though the most recent and thorough reconstruction of his position (Stan 2016) indicates not. Mach at some moments perhaps. If this view were correct, then the question of whether the Earth or heavens rotate would be ill-formed, those alternatives being merely different but equivalent expressions of the facts.