There is a problem which Materialists face that is rarely talked about. (Materialism is the belief that only matter and physical forces exist, i.e. that all disciplines are really a form of applied physics.) And the problem is a fairly basic one: what is an individual person?
In one sense this is a silly question because we all know. But the problem, for Materialists, is that what we all know directly contradicts Materialism. And it contradicts it because what makes a particular person that person transcends the particular matter which they’re made of. A materialist denies that there is anything can transcend the particular matter; all that exists are sub-atomic particles and a few forces acting on them. How, then, could a materialist possibly define what a person is?
This is an especially hard problem over time, since the matter which makes up a particular body changes through the years. All proteins, fats, sugars etc. get recycled by the body in its process of continual renewal. Even more of a problem is that a person starts off weighing less than ten pounds and usually ends by weighing over 100, often quite a bit more than 100 pounds. By adulthood their original matter is largely long gone, and any matter which by chance is the same is a tiny fraction of the original. Other changes such as larger muscles, longer hair, shorter hair, losing a limb, growing extra teeth, and many other changes significantly change the physical configuration of the matter. Neurons in the brain are constantly being made and new synaptic connections forming and others going away. Neither the particular matter nor the shape of the matter can be used to define a person. And according to the Materialist, nothing else exists.
There’s even a further problem that Materialists face in defining people: if the only real thing are sub-atomic particles and forces, there isn’t a good way to distinguish between the person and the chair he is sitting on. Individual molecules have inter-molecular attractions, but so do the molecules in the person and the molecules in the chair. The wood is a different density than the person’s skin and muscles, but those are a different density than the person’s bones. And if this is hard, what about when two people shake hands?
In my experience, when you point this out to a Materialist, their reaction is to get annoyed and say, “come on, you know what I mean.” Or, “and yet I can reliably tell what is a person and what isn’t.” I’ve never understood why it is supposed to be an argument in the Materialist’s favor that in practice not even he believes the nonsense he’s saying.
Chris Lansdown 
I have found that to be one of the most common errors in human reasoning:
1. I have a theory M which entails that P is impossible
2. And yet P is possible
3. ∴ M & P are not really contradictory
Materialists very commonly do this. They somehow seem to think that their ability to ACCESS PHENOMENA IN EXPERIENCE cannot count against materialism as a theory, even though their materialism entails either that there cannot be such phenomena or they could not be accessed or known. They simply “know” materialism is right, and no matter what counter-evidence is submitted, they will insist that materialism is not refuted, because a materialist explanation of the phenomenon is possible—in principle or as a “just so” story if all else fails.
Hence the popularity of “emergent properties.” A thing which cannot do a thing that we need it to do (to preserve our theory) is said to magically acquire the ability to do the needed thing. How? Unknown: it just “emerges.”
Doesn’t materialism rule out minds? Not at all! Minds “emerge.” Problem solved. Except, you know, not.
Personal identity? Why, it also is based in matter (so it doesn’t count against materialism); it just happens to “emerge” from the matter!
How does “emergence” answer anything? NO MATTER! NEVERMIND!
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I think a useful starting point is to consider what can be known and demonstrated. Materialism occupies a theoretical space that is amenable to investigation, which makes claims from this perspective convincing where scientific methodology is considered convincing. The notion of “person” is conceptual, changes across time and place. The way we understand personhood subjectively may not reflect the objective reality of cells, chemical reactions, bacteria etc. that make us possible. If I believe I’m made of hard light and understand myself this way, it doesn’t make it so. The fact that we shed and generate cells over time doesn’t pose a problem for materialism. Sensory information travels from the tip of your finger to your brain from infancy to death, barring injury. There’s something in the 10 to 100+ pounds weight argument that implies people stay the same over time but of course they develop and change. Those who are unfortunate enough to suffer brain damage can become quite different people, which points to the importance of the brain in personhood. I don’t pretend to understand the physics around physical resistance but I don’t think my lack of understanding in any way undermines whatever explanations there are. What prevents your hand from merging with someone else’s is an interesting question. Have you done any investigation into why?
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I’m not a materialist. But here’s one way a materialist might think about a person (Strawson, for instance). A person is just the kind of entity to which both psychological predicates and physical predicates apply. There is no difference between the “I” who is 5’11” tall and the “I” who is in pain. In either case, “I” refers to the same entity. You identify the “minds” of others (though we do not have to admit of the mind as some independent, ontologically distinct entity), because you can ascribe psychological states/predicates to them on the basis of their behaviors.
Psychological predicates have an essentially dual use. They are self-ascribable on the basis of the first-person experience; they are other-ascribable on the basis of third person observation. But the meaning of the psychological predicate is exactly the same in both the first-person use and the third-person use.
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