 Kepler’s causal approach is above all present in his Epitome, a voluminous work which exercised a considerable influence on the later development of astronomy. In the second part of Book 4, he deals with the motion of the world’s parts. Not the two first laws but rather the third law, which he had recently announced in his HM, is Kepler’s starting point; for this law, rather than a calculational device for the path of one planet, represents a general cosmological statement, and thus it is more convenient for his approach here. At the same time, it should be pointed out that the third law is not necessarily the best point of departure for a dynamical, causal approach to motion, as Kepler intends here; for, in comparison with the previous causal approaches, the question of the location of the cause of power responsible for the production of motion remains relevant. The spheres, which in the traditional view transported the planets, had been abolished since the time of Tycho. Furthermore, Kepler is clearly against the “moving intelligences” of the Aristotelian tradition. The fact that the orbits are elliptical and not circular, shows that the motions are not caused by a spiritual power but rather by a natural one, which is internal to the composition of matter. The planets themselves are provided with “inertia”, a property, as Kepler understood it, that inhibits motion and represents an impediment to it. The motive power (vix motrix) comes indeed from the Sun, which sends its rays of light and power in all directions. These rays are captured by the planets. Kepler, however, tries to explain this behavior of the planets less through astrology and much more through magnetism (a physical phenomenon which was by no means clearly understood in his time). Firstly, the Sun rotates and, by so doing, sets in motion the planets around it. Secondly, since the planets are poles of magnets and the Sun itself acts with magnetic power, the planets are, at different parts of their orbits, either attracted or repelled; in this way the elliptical path is causally produced. Kepler partially gives up the mechanical approach by postulating a soul in the Sun which is responsible for its regular motion of rotation, a motion on which, finally, the entire system depends. In fact, the planets are also supposed by Kepler to rotate and are therefore provided with “a sort of soul” or some such principle which produces the rotation.