Materialism as a metaphysical perspective fails on countless levels. Nowhere is that failure more clear than in the understanding of the mind and the brain. The central failure of materialism in philosophy of the mind is the representation problem.
Mental representation is a big topic and has been a fertile issue in philosophy, especially over the past two centuries. I’ll focus on the role of mental representation in abstract thought, which is where the inadequacies of materialism are most obvious.
Thoughts may be divided into thoughts about particulars and thoughts about universals. Thoughts about particulars are thoughts, including perceptions, imagination, memory, etc., about particular objects in our environments. Thoughts about my coffee, or my car, or my family would be thoughts about particulars.
Thoughts about universals are abstract thoughts, and are thoughts about concepts. Justice, mercy, logic, mathematics, etc., are abstract thoughts.
For a materialist, all thoughts are generated by the brain. All that exists is matter, as understood by physics and chemistry. Thus, all thoughts, for the materialist, are generated purely physically, by neurons, neurotransmitters, action potentials, etc. So when we think about a particular object, that thought must somehow actually be a physical thing — a molecule or a relationship between molecules, etc. But of course, if I think about a particular thing — my cat Tabby, for example — my actual cat Tabby isn’t in my brain, so the materialist would say that my cat Tabby is somehow “represented” in my brain, and that representation constitutes the thought, without (immaterial) remainder. In the materialist view, all thought is, boiled down, matter of some sort, or is at least wholly represented in matter.
For thoughts about particular objects, this materialist scheme is not entirely implausible. For some aspects of visual perception, for example, there is a mapping of the visual field from the retina to the cortex, so that an image (of sorts) is represented in the brain as a field of neurons that are activated in a pattern. One might say that the pattern is the representation of the visual image. This still leaves much to be explained, but at least it is not utterly implausible to say that a thought about a particular thing — for example, a perception of my cat Tabby — is a representation in my brain. We still have no scientific (or metaphysical) explanation as to how this neuronal pattern actually becomes the thought, of course. But mental representation may provide a real level of explanation for thought about particulars.
But abstract thought is different. Consider a thought about justice. Justice is a concept, not a particular thing existing in the physical world. The materialist must ask: how can a thought about justice be represented in the brain? It certainly can’t merely be a mapped field in the cortex — justice has no shape or physical pattern, unlike my cat Tabby. A materialist would no doubt say that, like perception of particulars, thought about justice is represented in the cortex. But note carefully what representation means: a representation is a map of a thing. It presumes the existence, in the physical world, of that which it maps. A representation of a city — a map — presumes the city. A representation of my cat presumes my cat. And here’s the problem: a representation of my thought about justice presumes my thought about justice. So representation cannot provide any final explanation for abstract thought, because the representation of an abstract thought, even if it exists, presupposes the abstract thought itself.
As an example, let us suppose that a certain pattern of neuronal activation in my cortex were shown to represent my thought about justice. Obviously that pattern is not my thought about justice itself — justice is a concept, not a bunch of neurons. And if that pattern of neuronal activation represented my thought about justice, it must map to my thought of justice, which presupposes my thought about justice and thus cannot explain it.
Succinctly, mental representation of abstract thought presupposes abstract thought, and cannot explain it. It is on abstract thought that materialism, as a theory of mind, flounders. Abstract thought, classically understood as intellect and will, are inherently immaterial. Any representation in the brain of abstract thought (while it may exist) necessarily presupposes abstract thought itself, which must, by its nature, be an immaterial power of the mind.
The human mind is a composite of material particular thought and immaterial abstract thought. Interestingly, modern neuroscience supports this view. Perception of particulars maps with precision to brain anatomy, but abstract thought is not mapped in the same way. Material powers of the brain are ordinarily necessary for exercise of abstract thought (e.g., you have to be awake to think about justice), but matter is not sufficient for abstract thought.
Abstract thought is an immaterial power of the mind.