- For a long time philosophers thought material objects were unproblematic. Or nearly so. There may have been a problem about what a material object is: a substance, a bundle of tropes, a compound of substratum and universals, a collection of sense-data, or what have you. But once that was settled there were supposed to be no further metaphysical problems about material objects. This illusion has now largely been dispelled. No one can get a PhD in philosophy nowadays without encountering the puzzles of the ship of Theseus, the statue and the lump, the cat and its "tail complement", amoebic fission, and others.
- These puzzles have to do in part with what material objects there are. Which matter-filled regions of the world contain material things, and which don't? Is there something made up of your upper half and my lower half, for instance – a scattered object whose mass is something between my mass and yours? Come to that, is there such a thing as your upper half? What we say about the puzzles involving ships, statues, cats, and amoebas will depend largely on how we answer this question.
- The most common answer is that material objects are abundant. Every matter-filled region of space, or spacetime, contains at least one. We simply ignore most of them for practical purposes. Many philosophers assert this without argument, as if it were completely obvious. Others take it for granted without mentioning it, as if there were no alternative. A few take the other extreme: material objects are sparse. Most notably, "Van Inwagen (Peter) - Material Beings" (1990) argues that the only material objects are elementary particles and organisms.
- It is hard to find a serious defense of what we might call "folk ontology": the view that there are dogs and tables and planets and all the other material objects that we have names for (more or less), but no arbitrary, gerrymandered objects. It isn't hard to see why. There seems to be nothing that all and only those regions that folk ontology says are occupied by material objects have in common. (Apart from the fact that we think of them in that way, of course; but that couldn’t account for the ontological difference.)
- Merricks defends a view close to van Inwagen's, though his arguments are very different (they resemble those of Wheeler – Persons and their micro-particles. Noûs 20: 333-349, 1986). The only material objects, he says, are those with "non-redundant causal powers" – those that cause things not caused by their parts. There are human beings. (That's what we are – though why Merricks thinks so is unclear.) There are also microscopic particles – "atoms" for short. Whether there are any other material objects is uncertain. In any case there are no ships or statues or lumps of clay or arbitrary undetached parts of cats. Merricks calls his view eliminativism.
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