St. Albert the Great and Robert Grosseteste on the nature and causes of comets

Addressing a subject which has received very little attention, this article explores the interpretations of comets offered by St. Albert the Great (c. 1190–1280) and Robert Grosseteste (1168–1253). It shows how, despite prima facie convergences between the two 13th-century bishops concerning the nature and causation of comets, there are nonetheless several previously unobserved subtle differences between them. For Grosseteste the celestial bodies (i.e. the stars and the planets) are the primary, and indeed sole, efficient causes of cometary phenomena, serving to draw up rarefied matter to the upper atmosphere whereupon it is inflamed as it is assimilated to the celestial nature itself. For Albert, by contrast, while the celestial bodies may help to stir up combustible vapours within the atmosphere, and at times precipitate their ascension to the heavenly vault by means of their motion and conjunction, it is not always the case that a comet arises as a result of the direct efficient causality of the celestial bodies.