Prime Matter

One of the more controversial and difficult topics in Aristotelian/Thomist philosophy of nature is that of prime matter. According to A&T, material substances are in some sense "composed" of substantial form and prime matter. Prime matter is somewhat mysterious stuff. Thomas tells us (in De Ente et Essentia) that it cannot be defined or understood in itself but only in relation to form and the composite substance. Thomists often refer to prime matter as "pure potentiality", as though it had no actuality at all. Moreover, prime matter is supposed to play some central role in individuating material substances (that is, in grounding the numerical distinctness of distinct members of the same infima species), even though prime matter is not itself an individual, and even though all the world's prime matter is in some sense numerically one. How to make sense of all this?
 
In my own view, the primary role of prime matter is that of individuation Two "packets" of prime matter are primitively, fundamentally numerically distinct. Their distinctness is not grounded in anything else. Thus, prime matter has something actual (even necessary) about it--namely, its numerical identity to itself and its numerical distinctness from all other packets of prime matter (whether actual or merely potential, past, present, or future). In addition, the mereological facts about packets of prime matter (i.e., which packets are proper parts of which, which pairs of packets overlap) are themselves necessary truths (and hence, actual truths). Prime matter is a mass of gunky bare particularity. There are no atomic bits of prime matter--each packet of prime matter is divisible into smaller bits.
 
Thomas explains, in De Ente, that it is signate matter that individuates--prime matter considered under "determinate dimensions". What can this mean? Some commentators suppose that packets of signate matter are individuated by their accidents of spatial extension (shape, size, location). However, this leads to a vicious circularity: signate matter individuated by quantitative accidents of space, accidents of space individuated by composite substances, and substances individuated by signate matter. I would propose that a packet of signate matter is simply a packet of prime matter, and that packets of signate/prime matter are primitively individual. Remember that prime matter can only be understood relationally. So, a packet of signate matter is simply some prime matter considered as the individuator of some quantitative accident of space. It is the packet of matter that individuates the accident, not vice versa. But it is in relation to that individuated accident that we are able to define and understand a particular packet of prime matter. Individuation is a metaphysical relation, while understanding is a rational or epistemic relation. The packet of prime matter is metaphysically prior to the spatial accident, but the spatial accident is prior in the order of understanding.
 
St. Thomas does not say this, but I think that the "determinate dimensions" of a packet of signate matter must include a temporal dimension as well as a spatial one. That is, I propose that all packets of prime matter exist only for a single instant. This is because substantial form is the principle of rest/motion, and so it is also the principle of persistence. Prime matter as such cannot persist.