He argues against the view I think plausible that persons are just their brain (or actually some temporal stage of their brain). His argument is that views like mine must explain how it is that we can have properties that are not instantiated in the animal or physical brain that is co located with our selves when both things share the same constitution (made of the same stuff). There must be both an animal and a person sharing the same body, he argues, that we must explain how it is that there is much similarity between us without being the same thing and why we posit different properties despite the similarities. When I think a thought, does the animal that occupies the same spacetime region as me (or incorporates me if I am my brain) think the same thought or a different but qualitatively identical thought or no thought at all? If the animal no longer exists and was replaced by me, the person, we must explain where that animal went.
This is just a version of the grounding problem. Here, I can only respond that Karen Bennett has argued that the grounding problem is not really a serious problem because those who believe in coincident objects can simply posit the existence of (de se) modal properties as brute facts about things' having properties not grounded in their physical constitution. I find Bennett's argument very persuasive.
But I think I can also offer a tu quoque against Olson's view (i.e., that the same weakness he claims is a feature of the view I favor is a feature of his view as well). Oslon's view is the animalistic view. An animal comes into existence at the 2 week or so of gestation. However, there was an organism there before the animal; namely, that organism was a single celled organism (zygote) then became a multicellular embryo (without all the other criteria officially qualifying it as an animal). Is the animal which Olson think we all are the same being as the organism that came before it? If so then we would come into existence at the same time as it (not two weeks into gestation but about 48 hours after conception). If the organism came into being 48 hours after conception and the animal is identical with that organism then the animal must have also came into existence at that same time (by Leibniz's law of identity). If not, what happened to that organism? Was it replaced or does it exist coincidentally with the animal? So similar problems plague Oslon's view I believe. But it doesn't stop there. What of the oocytic view? This view is just that we are the original egg in which was fertilized by some sperm. Since the egg is about 1000 times larger than the sperm, the joining of both cells may be seen as the egg simply incorporating the sperm and changing a bit in terms of mass etc instead of going out of existence along with the sperm and both being replaced by a new being (the zygote). But the oocytic view is clearly preposterous.
