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. (shrink)

We will examine Kepler's use of a relation between velocity and distance from a centre of circular motion. This relation plays an essential role, through a derivation in chapter 40 of the Astronomia Nova, in the presentation of the Area Law of planetary motion. Kepler transcends ancient and contemporary applications of the distance-velocity relation by connecting it with his metaphysical commitment to the causal role of the Sun. His second main innovation is to replace the astronomical models of his predecessors (...) with an account that derives trajectories from physical principles. In this paper we restrict our attention primarily to the first 40 chapters of Kepler's Astronomia Nova. Hence, we do not discuss Kepler's discovery of elliptical planetary orbits. (shrink)