Van Inwagen’s General Constitution Question (GCQ) is to provide necessary and sufficient conditions for the xs to compose y in non-mereological terms. Van Inwagen is skeptical of whether the GCQ has any non-trivial answer. Here is a reason to think he’s right to be skeptical.
It might well be true that atomism is necessarily true, namely that every object has a simple part. But given atomism, the following is true:
- Necessarily, z is a part of y if and only if there are xs that include z and that compose y.
(Proof: The right to left part is trivial: if the xs compose y, they are parts of y. Conversely, suppose z is a part of y. Let the xs consist of z and all the atomic parts of y that aren’t parts of z. It is easy to see that the xs compose y.)
Thus, if atomism is true, an answer to GCQ will provide a non-mereological account of all of mereology. That seems to be too much to expect.
Even if atomism is true, an answer to GCQ will provide a non-mereological account of the parthood of all wholly non-gunky objects (where an object is partly gunky provided that it has some part that has no simple parts). That is also more than we should expect.
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