Since prime matter is itself a concept which is not known to all, it is useful to begin with a consideration of what it is and why there are difficulties involved in knowing it. (footnote about Physics 1 and how it lays out the discovery of prime matter) The concept of prime matter first arises when one is analyzing the principles of change. Whenever some change occurs, something must remains and something must pass away. For example, when a sick man becomes healthy, the illness ceases to be and the health begins to be, but the man exists both before and after the change. So also in any change there will something which persists through the change. If it were not the case that something persisted, then one could never say that any thing changed, but just that things sometimes exist and sometimes do not exist. Granting the existence of change, something must underlie it. To come to prime matter, one must see that this is true of both substantial change and accidental change. Accidental changes refer to the coming to be or passing away of accidents: for example, a change in size, place, or some quality. In such cases, it is always a substance which underlies the changes. Yet when a change occurs from one kind of substance to another, then something beside substance must underlie this change. This is what Aristotle calls the underlying nature, and what St. Thomas calls materia prima or first matter. (footnote about how I will use the phrase "prime matter" since that is the typical English rendition, although it seems first matter is better translation)
After resolving to the existence of such a principle, there are further things that can be said about it. Just as a substance is not in a certain place on account of being that substance, but is potentially in any place; so also it is true that prime matter, not being itself a substance, is potentially any substance. From this, prime matter is said to be pure potency with regard to every substantial form.
[It looks like I might have to do some restructuring of the outline. I next wanted to talk about how prime matter is not a principle of our knowledge. I was thinking that follows easily enough from it being pure potency, but without at least a brief account of how we abstract, it's going to be difficult to express the problem.]
[There's also the bit in the outline about how prime matter is a principle of individuation. It seems that I could talk about that here or later. That is a rather difficult topic all on its own... I do want to talk about how we know singulars though, and why God's knowledge of them has to be different from that. Especially look at the fact that we know singulars through our senses and how God does not have senses. Yes, this will do for now.]
[One more comment, the stuff written on theology can probably go even before this section. Also, there's no reason to bring up Berkeley: the existence of prime matter is sufficiently established. But it will be helpful to bring up Timaeus' position later on.]