Neuroscientists who work on the human brain seldom mention free will. Most consider it a subject better left, at least for the time being, to philosophers. Meanwhile, their sights are set on discovering the physical basis of consciousness, of which free will is a part. No scientific quest is more important to humanity. Everyone — scientists, philosophers, and religious believers alike — can agree with the neurobiologist Gerald Edelman that “[c]onsciousness is the guarantor of all we hold human and precious. Its permanent loss is considered equivalent to death, even if the body persists in its vital signs.”

The physical basis of consciousness won’t be an easy phenomenon to grasp. The human brain is the most complex system, either organic or inorganic, known in the universe. Each of the billions of nerve cells (neurons) composing its functional part forms synapses and communicates with an average of ten thousand others; each launches messages along its own axon pathway using an individual digital code of membrane-firing patterns. The brain is organized into regions, nuclei, and staging centers that divide functions among them. These regions respond in different ways to hormones and sensory stimuli originating from outside the brain, while sensory and motor neurons all over the body communicate so intimately with the brain as to be virtually a part of it.

Untitled, a collage on paper by Frederick Sommer. Courtesy the Frederick & Frances Sommer Foundation and Bruce Silverstein, New York City

Of the 20,000 to 25,000 genes in the human genome, half participate in some manner in the prescription of the brain-mind system. This amount of commitment has resulted from the most rapid evolutionary change known in any advanced organ system of the biosphere. It entailed a more than twofold increase in brain size across 3 million years, from 600 cubic centimeters in the australopith prehuman ancestor to 900 cubic centimeters in Homo habilis, thence to about 1,400 cubic centimeters in modern Homo sapiens.