How are human functions such as movement, learning, thought, and reflexive action regulated by the body? For a very long time, scholars divided these processes into two large categories: those controlled by the brain and those controlled by the mind. According to this duality, intellectual processes such as critical thinking and memory were a function of the mind, an immaterial entity that was not subject to scientific study, but was to be understood in terms of philosophical analysis. The brain, on the other hand, was thought to be a concrete, physical entity responsible for phenomena such as muscular activity that could be investigated by scientific means. During the Middle Ages, for example, many scholars assigned the control of higher intellectual processes to an immortal and invisible soul and the control of muscular behavior to a tangible and mortal soul that could be studied scientifically.

Studies of...