Natural motions are those that a body exhibits when unimpeded. Violent motions occur when a body is displaced from its natural resting place. According to Aristotle, celestial bodies naturally move in circles or combinations thereof. Since it was thought that the ether in no way hindered this motion, it was concluded that these bodies do not exhibit violent motions. Bodies composed of ordinary matter behave quite differently and in a manner intimately tied to the structure of the sublunar world.
The terrestrial region of Aristotle's universe was composed of four concentric areas, each the natural place for one of the four elements. When displaced, it was believed that each element naturally moves rectilinearly toward its concentric ring (if unimpeded). The outermost ring was the natural place of fire, below that the ring of air, below that the ring of water, and below that the ring of earth. Aristotle attributed different degrees of heaviness or lightness to the basic elements to explain their tendency to seek their natural places. The element earth was deemed absolutely heavy. As such, it naturally moved downward toward Earth's center from the regions above. Similarly, the absolute lightness of fire caused it to rise from below to its natural place above the ring of air.
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