 To see that the problem of the interpretation of spatiotemporal quantities as absolute or relative is endemic to almost any kind of mechanics one can imagine, we can look to one of the simplest theories – Aristotle’s account of natural motion (e.g., On the Heavens I.2). According to this theory it is because of their natures, and not because of ‘unnatural’ forces, that that heavy bodies move down, and ‘light’ things (air and fire) move up; it is their natures, or ‘forms’, that constitute the gravity or weight of the former and the levity of the latter. This account only makes sense if ‘up’ and ‘down’ can be unequivocally determined for each body. According to Aristotle, up and down are fixed by the position of the body in question relative to the center of the universe, a point coincident with the center of the Earth. That is, the theory holds that heavy bodies naturally move towards the center, while light bodies naturally move away.