Gersonides rejects the basic tenets of the Aristotelian theory of natural motion, and offers an alternative original account. He does not introduce his theory systematically nor argues openly with Aristotle, but conveyes his ideas through a subtle and sophisticated work of exegesis. Developing ideas of Aristotle, Averroes and Themistius, he eventually ends up with a different theory. Gersonides presupposes neither that there are absolute natural places, which are the final causes of natural motion, nor that there is inclination in the element, which is the efficient cause, nor that motion in a straight line is natural to the elements. He explains the motion of a body only in terms of its relations to adjacent bodies in contact with it. The body rises when the medium surrounding it is heavier and sinks when it is lighter. The distance from the innermost celestial sphere, rather than "natural places," determines the heaviness or lightness of the medium and the structure of the sublunar region.