 Now, a body has as many relative motions as there are bodies, but it does not follow that all are equally significant. Indeed, Descartes uses several different concepts of relational motion. First there is ‘change of place’, which is nothing but motion relative to this or that arbitrary reference body (II.13). In this sense no motion of a body is privileged, since the speed, direction, and even curve of a trajectory depends on the reference body, and none is singled out. Next, he discusses motion in ‘the ordinary sense’ (II.24). This is often conflated with mere change of arbitrary place, but strictly it differs because according to the rules of ordinary speech one correctly attributes motion only to bodies whose motion is caused by some action, not to arbitrary relative motion. (For instance, a person sitting on a speeding boat is ordinarily said to be at rest, since ‘he feels no action in himself’.) This distinction is important in some passages, but arguably not in those that we discuss. Finally, he defined motion ‘properly speaking’ (II.25) to be ‘the transference of one part of matter or of one body, from the vicinity of those bodies immediately contiguous to it and considered as at rest, into the vicinity of [some]  others.’[2]  Since a body can only be touching one set of surroundings, Descartes argued (questionably) that this standard of motion was unique.