  Accepting the observational under-determination of motion understood  as mere change of relative position, and finding absurd the  consequence that there is no genuine motion in the world, Leibniz  denies the premise that there is nothing more to motion than relative change of position. He thus maintains that rather than grant that  there is no real motion, “in order to say that something is  moving, we will require not only that it change its position with  respect to other things but also that there be within itself a cause  of change, a force, an action” (G IV.396/L 393). Intuitively,  Leibniz’s suggestion is that genuine motion requires, in addition to  relative change of place, a cause of that relative change. To return  to our earlier example, if Train A moves relative to Train  B, we should say, according to Leibniz, that Train  A really moves if and only if, it is the active cause of  their relative motion. Since, for Leibniz, being an active cause, or  locus of force, is a non-relativistic property attributable to  individual bodies, it is in principle capable of breaking the  “equivalence of hypotheses” and thus grounding true or  genuine motion. 