Kepler's theory of force and his medical sources

Johannes Kepler (1571-1630) makes extensive use of souls and spiritus in his natural philosophy. Recent studies have highlighted their importance in his accounts of celestial generation and astrology. In this study, I would like to address two pressing issues. The first is Kepler's context. The biological side of his natural philosophy is not naively Aristotelian. Instead, he is up to date with contemporary discussions in medically flavored natural philosophy. I will examine his relationship to Melanchthon's anatomicaltheological Liber de anima (1552) and to Jean Fernel's very popular Physiologia (1567), two Galenic sources with a noticeable impact on how he understands the functions of life. The other issue that will direct my article is force at a distance. Medical ideas deeply inform Kepler's theories of light and solar force (virtus motrix). It will become clear that they are not a hindrance even to the hardcore of his celestial physics. Instead, he makes use of soul and spiritus in order to develop a fully mathematized dynamics. © 2014 Koninklijke Brill NV, Leiden, The Netherlands.