Suppose somebody says “I think that personal identity is an illusion”. Granted, if they are Naturalists they might think that they must adopt such a commitment, since it is impossible for aggregates of matter to be ‘about’ other things, and yet personal identity or subjectivity is grounded in states of intentionality (about-ness).
However, think about what the statement above presumes. It presumes personal identity, for without personal identity one could not intelligibly issue the statement from the first person perspective, using the first person pronoun “I”. Who is it that doesn’t believe in personal identity exactly? Since we know indubitably by introspective reasoning and intuition that we are persons (subjects), our preferred system of Metaphysics should be able to account for ourselves as the kind of subjects we know ourselves to be. In the absence of a defeater, which it appears we do not have, we are justified in believing in our own personal identity, and if some metaphysical system cannot account for our own personal identity (Materialism, for example), then we have every reason to reject that metaphysical system. Sometimes Naturalists who are not, at bottom, idealists, will simply argue that perhaps one day Naturalism as a metaphysic will be able to account for consciousness or personal identity (the irreducibility of the first person perspective) in terms of something like a reduction to physical entities and phenomenons. I suspect the idealists are comfortable without that kind of reduction because they reject the principle of sufficient reason and are willing to accept irreducible states of first person perspective as brute facts with which we are presented. However, concerning the Naturalist who wants to make Naturalism into a dialectically intelligible system of metaphysics (for which the PSR is required) it seems they are, here, grasping at straws. This is the most embarrassing instance of Naturalists committing the ‘Naturalism of the gaps’ fallacy, and worse, they are committing it often without recognizing that it is not logically possible that Naturalism as a dialectically intelligible system of metaphysics can account for an irreducible state of subjectivity. No Naturalistic scenario will secure the veridicality of an irreducible state of subjectivity and affirm personal identity as something other than, say, psychological or ‘narrative’ continuity (neither of which account for the fundamental first person perspective from which all conscious experience arises).
Naturalism, at its best, can only convince us that we are not irreducibly subjective (subjects which cannot be merely an aggregate of objects), and that the first person perspective is illusory. However, that seems to be something which is not rationally affirm-able, since we would have to say “I am not a subject” or “I do not believe I am a subject.”
I think the argument from consciousness, or intentional states, is a great argument, therefore, against Naturalism taken as a system of metaphysics.