 Such determination, he says, can be accomplished by observing the causes of motion (that is, "forces") and not simply the apparent motions of bodies relative to one another (as in the bucket argument).As an example where causes can be observed, if two globes, floating in space, are connected by a cord, measuring the amount of tension in the cord, with no other clues to assess the situation, alone suffices to indicate how fast the two objects are revolving around the common center of mass.