This is just a progress report, a promissory note without much argument. I've been thinking a lot about naturalism and persons. Specifically, I've been thinking whether there is room for persons in a naturalistic ontology. One lemma that I've become convinced of is that if necessarily all persons are substances, so that if x is a person, then for x to exist is not a matter of something beyond x having some property or standing in some relation, then naturalism is false. In an earlier post, I gave an argument for this conclusion based on speculative physics, but now I am convinced that the conclusion holds independently of the speculative physics. Basically, the idea is that if naturalism holds, strong AI is true (it would be too weird if naturalism were true but minds had to be tied to a biology like ours), but if strong AI is true, then I suspect it is possible for a token computer program to be a person, and token computer programs are not substances (their existence is a matter of a computer having a particular state).
Moreover, it is plausible that finite persons are ontologically homogeneous: if one finite person is a substance, they all necessarily are. If this is correct, then if we are substances, naturalism is false.
Are persons substances? Are we substances? If we adopt an Aristotelian ontology, there are three alternatives to a person being a substance: she might be accident-like (e.g., a trope or a token relation), she might be the essence of a substance, or she might not exist. I take it that persons exist. The same kinds of thoughts that suggest that if naturalism holds, persons need not be substances, also suggest that if naturalism holds, then persons need not be essences of substances. So, on this kind of ontology, the question comes down to: Can persons be accident-like?
But consider the following thoughts: (a) if naturalism is true, then the best theory of personal identity will be a memory-based theory, (b) programs can seamlessly move between processors and even between computers, and (c) accident-like entities cannot move between substances. To me, these thoughts suggest that if naturalism holds, persons can't be accident-like, unless appearances are deceiving and moving from one body to another, or one computer to another, wouldn't involve a movement between substances. But the only way this could be is if the persons are accidents of some grand global substances, like the Cosmos, or Spacetime, or the Fields of a unified field theory.
Thus, assuming an ontology that has only substances and accident-like entities, the conclusion I draw is that if naturalism holds of persons, we are all modes (to use Spinozostic terminology) of one or more global substances. I doubt that on a sparse theory of properties and relations there will be enough modes to do the job. So the ontology will have to be one on which there are one or more global substances, of which everything else is a mode, and the modes are abundant. Moreover, since we have properties, this ontology will have to be one on which accident-like entities can be nested. I suspect that abundance will cause Unger-like problems with identifying who exactly we are, but I would like to have a better argument against such a Spinozistic ontology.
So this is where I am at right now in the argument: either some non-naturalistic account of persons is true, or a Spinozistic naturalistic ontology of one or more global substances, with nestable modes, probably abundant, holds. Of course I am convinced that the Spinozistic account is false (if only for ethical reasons: it doesn't do justice to the ethical importance of the body), but it would be nice to have a good ontological arguemnt here. With some modal imagination we might make progress: for instance we might think that even if in fact such an ontology holds, surely it would be possible to have persons apart from such an ontology, and this is enough to sink the naturalistic account. But I would rather not rely on modal imagination.
There is a lot of detail here that can be questioned. But the basic idea is, I think, sound: further progress on the question of whether the existence of persons is compatible with naturalism is going to be a matter not just of metaphysica specialis but of metaphysica generalis (i.e., of ontology).