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.