 Newton also points out that the height that the water climbs up the inside of the bucket provides a measure of the rate of rotation of bucket and water: the higher the water rises up the sides, the greater the tendency to recede must be, and so the faster the water must be rotating in the mechanically significant sense. But suppose, very plausibly, that the measure is unique, that any particular height indicates a particular rate of rotation. Then the unique height that the water reaches at any moment implies a unique rate of rotation in a mechanically significant sense. And thus motion in the sense of motion relative to an arbitrary reference body is not the mechanical sense, since that kind of rotation is not unique at all, but depends on the motion of the reference body. And so Descartes’ change of place (and for similar reasons, motion in the ordinary sense) is not the mechanically significant sense of motion.