This volume provides a discussion of John Philoponus’s treatise on prime matter (Contra Proclum XI 1-8) which defines prime matter as three-dimensional extension. The author traces the development of this definition in Neoplatonic thought with emphasis on the philosophical problems it was meant to solve. Its significance for the understanding of Aristotle’s category theory, for the physical explanation of growth and diminuition, and for the notion of formlessness are discussed in detail. Among other things this study reveals that Philoponus’s insights can be regarded as an imaginative response to Plotinus’s views on matter.