Parts and ownership

Some Aristotelians believe the following thesis: When a bit of matter comes to be a part of a substance, it ceases to exist. I.e., the bit of matter comes to be a part of a substance in the way in which a horse comes to be a corpse--the horse and the corpse are distinct entities, the corpse originating in the horse. If they believe this thesis, they have to give some explanation of why particles ingested can still be scientifically detected. They will do this either by saying that the particles no longer exist literally, but "virtually" do, or by saying that new particles just like the old ones come to exist out of the old ones, except the new ones have the essential property of being a part of substance that they have joined. There are two different reasons why one might believe such an outlandish thesis: (1) because one doesn't believe in parthood (Patrick Toner and Alexander Pruss incline in this direction), or (2) because one thinks that parts receive their identity from the whole (there is some textual basis in Aristotle for this).

I want to offer two arguments for this thesis, one metaphysical and Aristotelian, the second ethical and eccentric. I find the first plausible, and much less so the second, but the second is kind of neat, so I'll give it, too:

• Matter receives its identity--that which makes it be the entity that it is--from the substance that it makes up. Therefore, if a bit of matter x is a part of substance A and a bit of matter y is a part of substance B, then if A is distinct from B, it follows that x is distinct from y. Therefore, no bit of matter can be a part of two substances. But everything that exists is a substance or a mode, relation, trope, accident or the like. A proper part of a substance is not a mode, relation, trope, accident or the like. Hence, if a substance has a proper part, that proper part is a substance. But the matter of a proper part A of a substance B would then be a part of both substance A and substance B, which contradicts the thesis that a bit of matter can't be a part of two substances.
• This argument uses two main assumptions:
  1. Without receiving special normative power from you or some higher authority, the only way I can by myself make an item x that you presently literally own to cease to be literally owned by you is by destroying x.
  2. It is impossible for one person to literally own a part of another.
  Suppose now you literally own a carbon atom x (e.g., it's part of a steak that you own), and I without having received any special normative power eat the atom x so that it becomes a part of my body. By (2), I have made it happen that you no longer own x. But by (1), it follows (assuming I have received no grant of authority or the like), the only way I could have done this is by destroying x. Hence, when an atom becomes a part of my body, it is thereby destroyed.