Neurosurgeon Dr. William Penfield, after a lifetime of mapping brains, posited that the mind is separate from the brain:
His third line of reasoning was the following: He would ask people to move their arm during the surgery. So he’d be playing around with their brain. And he’d say. “Whenever you want to, move your right arm.” The person would move their arm.
And, once in a while, he’d stimulate the part of the brain that made the arm move. And they moved their arm also when he did that. And then he would ask them, “I want you to tell me when I’m making your arm move and when you’re moving your arm without me making you do it. Tell me if you can tell the difference.” And the patients could always tell the difference.
The patients always knew that when he stimulated their arm, it was him doing it, not them. And when they stimulated their arm, they were doing it, not him. So Penfield said, he couldn’t stimulate the will. He could never trick the patients into thinking it was them doing it. He said, the patients always retained a correct sense of agency. They always know if they did it or if he did it.
So he said the will was not something he could stimulate, meaning it was not material.