When are Objects Parts?
Van Inwagen (Peter)
Source: Philosophical Perspectives, Vol. 1, Metaphysics (1987), pp. 21-47
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Author’s Introduction

  1. That there are deep and intractable metaphysical problems about material objects is evident from the unresolved antinomies and paradoxes involving material objects.
  2. The best known of these are about artifacts, the puzzle of the Ship of Theseus1 being the best known of all. Others, at least as worthy of remark, involve living organisms. If a cat's tail is cut off, for example, it seems natural to describe this episode in words that appear to imply that the cat becomes identical with a former proper part of itself – a violation of the attractive modal principle that a thing and another thing cannot become a thing and itself.
  3. Or, talking of cats, consider a cat that is composed at t of certain atoms arranged in a certain way; it is at least logically possible for those very same atoms to be arranged in exactly the same way at some later time, and then to compose a different cat; but that apparently implies that certain small material objects can compose one large material object at t, and, even though arranged in precisely the same way, compose a distinct material object later. How could that be?
  4. All, or almost all, of the antinomies and paradoxes that the philosophical study of material objects is heir to involve the notion of parthood. I believe, though I shall not argue for this thesis, that most of the great, intractable metaphysical puzzles about material objects could be seen to have quite obvious solutions by one who had a clear understanding of what it was for one material object to be a part of another.
  5. In this paper, I shall try to advance our understanding of this notion. I shall approach the concept of parthood in a somewhat indirect way. There is a mereological concept that I have found it easier to think fruitfully about than I have parthood. (By a mereological concept, I mean one that can be given a trivial definition in terms of parthood.) I call this notion composition.
  6. I give some examples. Suppose there is a house made entirely of bricks. Then those bricks compose the house. If there are such things as the north and south halves of the house, then those two halves also compose the house, and each of them is composed of certain bricks. The eastern third of the house, the western third of the house, and its middle third also compose it. Certain molecules compose the house. Certain atoms compose the house. Certain elementary particles compose the house. These examples should make it intuitively clear what composition is.
  7. The question that will be our main topic is this: Suppose one had certain non-overlapping material objects, the xs, at one's disposal; what would one have to do – what could one do – to get the xs to compose something? (I have found it to be heuristically useful to put the question in this "practical" way, in a way that invites the inquirer to contemplate various courses of action. But this way of asking the question must not be taken to imply that the xs are objects that human beings are able to manipulate. The xs may just as well be quarks or stars – or the members of some quite heterogeneous class of material things – as bricks or tinker-toy parts. The inquirer should, therefore, imagine himself to be omnipotent.) An interesting answer to this question must not, of course, involve mereological concepts. It would be no fair [way] to answer it by saying, "To get the xs to compose something, cause them to have a sum."
  8. I shall call this question the Special Composition Question.

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