This book offers an original account of one of Aristotle's central doctrines, his theory of material substance. Gad Freudenthal argues that Aristotle's concept of heat is a crucial but hitherto ignored part of this account. Aristotle's `canonical', four-element theory of matter fails to explain the coming-to-be of material substances (the way matter becomes organised) and their persistence (why substances do not disintegrate into their components). Interpreters have highlighted Aristotle's claim that soul is the active cause of the coming-to-be and persistence of living beings. Dr Freudenthal draws on dispersed remarks in Aristotle's writings, to argue that Aristotle in parallel also draws on a comprehensive `naturalistic' theory, which accounts for material persistence through the concepts of heat, specifically vital heat, and connate pneuma. This theory, which bears also on the higher soul-functions, is central in Aristotle's understanding of the relationship between matter and form, body and soul.Dr Freudenthal aims not only to recover this theory and to highlight its explanatory roles, but also to make suggestions concerning its origin in Presocratic thought and in Aristotle's own early theology. He further offers a brief review of how later ages came to grips with the difficulties inherent in the received version of Aristotle's matter theory. This book is an important contribution to the proper understanding of a central Aristotelian doctrine, which straddles `chemistry', biology, the theory of soul, and metaphysics.