Materialists or physicalists (those who believe that every phenomena in the universe has an ultimate explanation in some material substance or some physical law) hold that mind is either something produced by the brain or is just a function of the brain, much like digestion is a function of the stomach.
Dualists believe that the mind is a completely distinct substance that somehow interfaces with the material brain but is not reducible to, or explicable in terms of, brain function.
One neuroscientist/neurosurgeon in the mid-20th century who started out as a materialist and, as a result of his professional experience, was moved to become a committed dualist was a man named Wilder Penfield who was one of the preeminent scientists studying the brain.
Contemporary neurosurgeon Michael Egnor writes about him in an essay in which he describes two lines of evidence which led Penfield to abandon materialism: Penfield .... pioneered the surgical treatment of epilepsy using stimulation and recording from the surface of the brain in awake patients undergoing brain surgery. This was possible because the brain itself feels no pain, and the scalp can be anesthetized with Novocain-like drugs to render the surgery painless. This surgery is still being done today.
Penfield was especially interested in the relationship between the brain and the mind. He began his career as a materialist and he ended it as a passionate dualist. He based his dualism on two observations:
1. There are no intellectual seizures: Seizures are sporadic electrical discharges from the brain and they cause a variety of symptoms, from complete loss of consciousness to focal twitching of muscle groups, sensations on the skin, flashes of lights or noises, smells, and even intense memories or emotional states.The question whether we have an immaterial mind (or soul) is not just an academic issue. If materialism is true, if we are purely material beings, then it becomes harder to justify a belief that there's anything about us that survives the death of our bodies. It also becomes harder to believe that we have free will, and thus moral responsibility and human dignity.
Penfield could record these electrical discharges from the surface of the brain. Penfield noted that there are no intellectual seizures. That is, there has never been a seizure in medical history that had specific intellectual content, or abstract thought. There are no mathematics seizures, no logic seizures, no philosophy seizures, and no Shakespeare seizures.
If the brain is the source of higher intellectual function, as is widely believed, why in medical history has there never been a seizure that evoked abstract thought? This fascinated Penfield, and he inferred quite reasonably that the reason there are no intellectual seizures is that abstract thought does not originate in the brain.
2. Free will cannot be simulated by stimulation of the brain: Part of Penfield’s research was to stimulate the motor areas of the brain, which caused patients’ limbs to move during surgery.
He was the first surgeon to map the motor areas of the brain in this fashion. In doing this, he noticed that patients always knew the difference between stimulated movements and movements that they freely caused themselves.
Penfield would ask patients to move their limbs freely whenever they chose, and he would (without telling them) stimulate their limbs to move. Patients always knew the difference between movements freely chosen and movements caused by the surgeon.
Penfield could never find a region of the brain that simulated free will. He concluded that free will is not in the brain — it is an immaterial power of the mind.
Ideas have consequences and the consequences of where we stand in the debate between dualism and materialism are profound.