Gersonides has at times been viewed as an essentially orthodox Aristotelian in his metaphysical views. This designation, however, has been challenged on a number of grounds. This paper examines the way in which Gersonides revises the traditional conception of hylomorphism by positing that matter can exist without form. Motivated by a desire to reconcile Aristotelian natural philosophy with the Ptolemaic astronomical model, formless matter is seen as a necessary entity to posit in order for his cosmological model to be coherent. (...) By allowing for this exception case, Gersonides turns hylomorphism from a metaphysical principle to a general, though not universal, principle of physics that can must be revised if other observational and philosophical commitments are to be maintained, which speaks to a distinct attitude toward Aristotelian metaphysics than is otherwise assumed to be the case for Gersonides and his contemporaries. (shrink)