 One obvious question pertains to how low such underlying levels might go. In fact there is considerable controversy concerning how to conceive the bottom rung of Aristotle’s hierarchy of matter. Aristotle believes that everything is made of earth, air, fire and water. These elements are defined by their possession of one of each of the two fundamental pairs of opposites, hot/cold and wet/dry. Aristotle also thinks that these elements can change into one another (On the Heavens iii 6, 305a14–35). If his analysis of change is correct, when some water changes into some air, there must be something underlying, some substrate, which persists through the change, initially having the essential properties of water (being wet and cold, on Aristotle’s view) and then later those of air (being wet and hot). The thing that underlies this kind of change cannot be any of the elements, since it must be capable of possessing the properties characteristic of each of the elements successively, capable of being first cold and then hot, for example. The traditional interpretation of Aristotle, which goes back as far as Augustine (De Genesi contra Manichaeos i 5–7) and Simplicius (On Aristotle’s Physics i 7), and is accepted by Aquinas (De Principiis Naturae §13), holds that Aristotle believes in something called “prime matter”, which is the matter of the elements, where each element is, then, a compound of this matter and a form. This prime matter is usually described as pure potentiality, just as, on the form side, the unmoved movers are said by Aristotle to be pure actuality, form without any matter (Metaphysics xii 6). What it means to call prime matter “pure potentiality” is that it is capable of taking on any form whatsoever, and thus is completely without any essential properties of its own. It exists eternally, since, if it were capable of being created or destroyed, there would have to be some even lower matter to underlie those changes. Because it is the matter of the elements, which are themselves present in all more complex bodies, it is omnipresent, and underlies not only elemental generation and destruction, but all physical changes. As a completely indeterminate substratum, prime matter bears some similarities to what modern philosophy has called a “bare particular” (see Sider 2006), although, not being a particular, it may have more in common with so-called “gunk” (see Sider 1993).