This paper examines the view held by Francesco Piccolomini (1523-1607) on the relation between prime matter and extension. In his discussion of prime matter in the Libri ad scientiam de natura attinentes Piccolomini develops a theory of prime matter that incorporates crucial elements of the viewpoint adhered to by the Neoplatonist Simplicius. The originality of Piccolomini’s undertaking is highlighted by contrasting it with the ideas found in Jacopo Zabarella’s De rebus naturalibus . The case of Piccolomini shows that, in order (...) to classify early modern metaphysical theories of prime matter, the category ‘prime matter as sheer dimensionality’ is indispensable. (shrink)