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 c