Convinced by Comparison: Lutheran Doctrine and Neoplatonic Conviction in Kepler’s Theory of Light

Abstract

Johanne Kepler’s investigation of the epistemology and ontology surrounding the nature of light bridged the gap between his Lutheran and Neoplatonic foundations and his revolutionary idea of a physical, causal astronomy. Kepler sought to find logically the “true cause” behind the virtus motrix (“motive power”) that moved the planets and determined their organization. He employed Lutheran regressus reasoning and merged Plotinian-Neoplatonic emanationism with his own empirical observations to form a theory of light, which he legitimized with analogy and exemplum reasoning. Though his observations forced him to reject the Neoplatonic idea of light as a virtus motrix, he demonstrated that light and the virtus motrix were two species of the genus of forces that attenuate with distance. These conclusions allowed Kepler to theologically, mystically, and empirically confirm the motion of the planets as the effects of a universal, physical law.