Kepler’s struggle with the problem of force obstruction

Abstract

The paper examines how the scarcely discussed problem of force obstruction contributed to shaping Kepler’s concept of force, that is, the question whether corporeal forces are blocked when an intermediate body is interposed between the force source and the target body. We explore the role played by this problem in Kepler’s works, focusing on his Optics, Foundations of Astrology, Astronomia Nova and Epitome. While the problem is instrumental in abandoning an initially physicalistic framework for astrology, Kepler attempts at different solutions when discussing the forces responsible for planetary motion. These forces are first modelled upon light, and then magnetism (and gravity). However, after considering several ways to get rid of the problem, he is eventually unable to decide why observable effects of obstruction are usually absent. So he accepts this absence at face value, similarly to Newton in his most relevant passages, allowing for a new ontological category that is physical on the one hand, but non-material on the other.