RomanJoe wrote:

A suppositum's ability to occupy new spatiotemporal locations presumes that it has a potentiality to do so. To have a potentiality for a new state of existence--i.e. existing at location x and potentially existing at location y--requires a metaphysical explanation. There must be some determinable yet undetermined metaphysical part of the suppositum which allows it to take on new existences. This is what the scholastic calls prime matter. Any attempt to replace it with physical laws or space just misses the point because such a replacement presumes that beings have a potential to take on new existences in spacetime.

Why do we need prime matter though? We can understand the motions of material beings through certain physical laws which help us to predict their movements.
I'm not sure it would be presumptuous to stop at physical laws. If physical laws give us a description of material reality,  and its motion within spacetime,  then it seems superfluous to appeal to prime matter in such a case.

FrenchySkepticalCatholic wrote:

Thus, they're not simples, are they? They wouldn't be sharing something. Form and matter are both present at the atomic level.

Don't forget that you can apply the form/matter distinction to any material thing.

True, if we affirm the existence of forms then we would need to posit something to individuate those forms (or else we couldn't even entertain the idea of beings sharing a nature), otherwise we would have monism. But would prime matter be necessary to explain this individuation? From what I understand, prime matter is supposed to explain why forms are cut up into distinct beings, instantiated in the physical world as things with potentialities. Thus prime matter explains why substance 1 is potentially on the table and substance 2 is potentially on the ground. If individuation needs explanation, however, why appeal to prime matter? Why not just appeal to space. That is, substance 1 is spatially one way while substance 2 is spatially another way. They both occupy different realms of space.  

Miguel wrote:

I see no competition whatsoever. We just found out that material things are more composite than we once thiught they were, but the medievals were obviously already aware that bodies were composed by many different things which themselves were material (like organs, even though organs wouldn't properly be considered substances). Particles are just further parts; if they exist (many scientists, especially in QM don't actually believe fundamental particles exist btw, sounds a little weird but that's the way it goes) they are also composites of form and matter. So where's the competition?

No doubt this is true. But the thrust of my objection is that we have seemed to have uncovered the basic underlying structure of all material beings, that is, the atomic and subatomic levels. Where prime matter was posited once to account for the possibility of change in all beings, now physical atomic matter is posited as the substrate by which substantial change takes place,with aubstantial change being the reorganization of atoms into different structures. Notice I'm not advocating an all out reductionism. I am open to the idea that structures of atoms can yield emergentist or epiphenomenal qualities that the individual  atoms themselves don't have (e.g. sensation, thought, unity).   

Miguel wrote:

I would say that the chief insight of hylemorphism is really the distinction between formal and material causes as explanations, and how the idea of "matter" not structured into any form would be a mere abstraction. Atoms aren't just "little blobs of matter", they only are what they are (atoms) because of how they are structured; they're matter informed by the form of atom. This would go all the way down to the most fundamental particles: at the bottom, they are all matter structured in a certain common form. How would they not be? What else would they be? Little building blocks of "matter"? No, they are -- like other material beings -- material matter informed by

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