 Newton was perfectly well aware that the notion of absolute space is not  unproblematic.[15]  For instance, if a body’s true motion just is its absolute motion, its motion with respect to space itself, then the imperceptibility of space would appear to render any detection of true motion difficult, if not hopeless. Indeed, how would we detect any body’s true motion on this view? We might be able to detect a body’s changing spatial relations with its neighbors, but not its changing relationship with space itself! Newton’s solution to this problem is ingenious. Under certain circumstances, we can detect a body’s true motion by detecting its acceleration. We can do so when the body is rotating or has a circular motion, for such motions often have detectable effects. This is one way of understanding what has become one of the most famous, if not infamous, experiments of the early modern period, Newton’s bucket. (He grew up in part on a farm in the English countryside, and often used deceptively simple examples.) If one takes an ordinary bucket and fills it with water, and then attaches a rope to the top of the bucket, one can then twist the rope and let it go in order to make the bucket spin. When the bucket full of water spins around, we can detect the water’s acceleration by its changing surface. As Newton puts it, using his laws of motion, the water endeavors to recede from the axis of its motion (hence its changing surface). But even an observer untutored in physics would grasp the importance of the water’s changing surface--that is, perceiving the effect does not depend on understanding the laws. In this way, despite the fact that Newton wishes to conceive of the water’s true motion as its absolute motion within space itself, which cannot be perceived, he shows his readers how they might detect the water’s true motion through its effects. Newton provides another simple experiment to illustrate a similar point. If two balls are joined together by a rope and then spun around, say over one’s head, then the changing tension in the rope will indicate that the balls are accelerated. Since any acceleration is a true motion—although not all true motions are accelerations, since a so-called inertial motion is not—this case indicates that we can detect a body’s true motion even though space itself is imperceptible. In this way, Newton did not merely develop an alternative to the Cartesian view of motion, along with its allied conception of space; he presented a view that could be employed to pick out some of the true motions of objects within nature. Once one has found a true motion, one can then ask what caused that motion (for Newton, as we will see, it is forces that are understood to cause motions). As the last line of the Scholium in the Principia indicates, that is one reason that Newton wrote his magnum opus in the first place.