Thanks Patrikios. It's always tricky to try to fit the "logic" arguments with the latest "science."
Going back to Sedley's Article "The Inferential Basis of Epicurean Ethics," I think there's a relationship here between Epicurus' decision to divide the entire physical universe between "matter" and "void," and the entire "feeling" universe between pleasure and pain.
Certainly there are many types of atoms and bodies, and many types of pleasures and pain. And Nature doesn't have an intelligent design which inscribes a book with those labels and blesses this decision to divide between matter or void and pleasure or pain. There's some kind of "proleptic" decisionmaking that tells us to do that rather than to try to categories - say - matter into five types and void into five types, and pleasures into five types and pains into five types.
It seems to me that this is the question of "universals" -- what is the justification for your categorization decision?
I'm gathering that Epicurus is resting his justification on the senses while also recognizing that he is reasoning. Like Jefferson says, the senses ultimately give us bodies moving through space. That's two things, and while it's important that ultimately the bodies be composed of indivisible atoms, it's really at the sense level that we divide things into to.
And on the feeling level it makes sense in the end to say that there's "desirable and undesirable" (pleasure and pain).
All of this revolves around the issue of whether this world of the sense is the real world, or whether there's a hidden "true world" set of forces or beings or forms behind it and directing it. Epicurus is showing that it is possible to construct a system that is totally consistent with our senses and feelings, but which operates without divine or other hidden forces directing it.
So from that perspective, I would expect that when Epicurus divided the universe between matter and void, the last thing that he would have accepted would be that there is some "third force" that sets everything in motion and keeps it moving.
So I am saying all that to agree with where I think you are going, which is that motion (or the capacity for motion) is something that is inherent in the nature of matter, and that all you need for motion is atoms (more than one) and space. I don't think Epicurus would have accepted conceptually that it is possible for there to be any force which ultimately does not derive/arise/emerge from one of the two categories - bodies and space.
And this is where I think there's a lot more discussion to be had of what "emergence" entails. If everything in the universe is composed of "atoms" and void, then *everything,* including motion/gravity/whatever, arises from the interaction of those two categories, with no other category possible or conceivable. if something "exists," it arises / emerges from "matter" and "void."
Sure it's possible to divide things into five or fifty categories of bodies or of feelings. But what Epicurus is working for, and what we need, is a manageable system of thought through which we can understand our place in the universe and from there how best to live. That's what analysis based on "atoms and void" and "pleasure and pain" gives us.