 The main philosophical objections to prime matter are that it is, at best, a mysterious entity that we cannot know anything about, since we never perceive it directly, but only the things it underlies. Of course, there can be good theoretical reasons for believing in things that we never actually see. No one has ever seen a quark, but we can still know things about them, based on the kind of theoretical work that they are required to perform. Still, Aristotle’s theory will be more parsimonious, if he can manage without positing such theoretical entities. At worst, prime matter is said to be outright contradictory. It is supposed to be capable of taking on any form whatsoever, and thus to have no essential properties of its own. The idea that it has no essential properties of its own seems to make it difficult for us to characterize it positively in any way: how can it be invisible, or eternal, or the ultimate bearer of properties, if these are not properties that belong to it essentially? Moreover, if it is what ultimately underlies all properties, it seems that it must be able to take on properties that are inconsistent with what we would like to be able to think of as its own nature: when Socrates turns blue, there is also some prime matter that underlies him, which also turns blue. But how can prime matter be simultaneous invisible and blue? To get around these problems, it looks as though proponents of prime matter will have to distinguish between two different kinds of property that prime matter has, or perhaps two different ways in which it has properties. There are its essential properties, which define the kind of entity that it is, and which it has permanently, and then there are its accidental properties, which it gains and loses as it underlies different sorts of thing. A worry about this solution is, if one can distinguish between the prime matter and its essential properties, this might suggest that there is a need for a further entity to act as the underlying thing for those properties, and then this further entity would need to have its own nature, and something to underlie that nature, and so on. It seems best to try to avoid such an infinite regress by insisting that prime matter can underlie its own essential properties, without being a compound of those properties and some further matter.