Aristotle’s Motivation for Matter Why does Aristotle make matter so central to his account of the natural world, making it a principle of nature and one of the four causes? Although there is considerable interest in how Aristotle conceives of matter, scholars rarely investigate why he thinks of it as fundamental to the natural world. Some simply ask why Aristotle thinks there must be matter. Other interpreters do not even agree that we should ask this question; they claim that Aristotle does not give reasons for needing matter because matter is an everyday notion we need not motivate. I think that in Physics I Aristotle gives us good reasons – perhaps even compelling ones – for thinking that matter is necessary for any understanding of the natural world. We, as interpreters, can use these reasons to understand what matter is for Aristotle, making progress where scholars have offered many incompatible interpretations. The first chapter of the dissertation presents my basic account of why Aristotle needs matter and what it is. I argue that Aristotle makes matter central to his natural philosophy because it is needed in order to understand change. Specifically, in order for there to be change, there must be something whose very nature is to undergo change. This is what matter is for Aristotle: the thing whose very nature is to undergo change. Since matter is picked out by its role in change, the very same thing will be matter and other things, based on what other roles it has. Just as the same person can be a doctor and a builder, so the same person can be a doctor and matter. This chapter also highlights the strength of my account by arguing against a rival interpretation of Aristotle’s motivation for matter, according to which matter is needed so that something persists through change. The second chapter argues for my interpretation through a close reading of Aristotle’s Physics I.