Although neither [the easy nor the hard] problem has been solved, neuroscientists agree on many features of both of them, and the feature they find least controversial is the one that many people outside the field find the most shocking. Francis Crick called it "the astonishing hypothesis"--the idea that our thoughts, sensations, joys and aches consist entirely of physiological activity in the tissues of the brain. Consciousness does not reside in an ethereal soul that uses the brain like a PDA; consciousness is the activity of the brain.
THE BRAIN AS MACHINE
SCIENTISTS HAVE EXORCISED THE GHOST FROM THE MACHINE NOT because they are mechanistic killjoys but because they have amassed evidence that every aspect of consciousness can be tied to the brain. Using functional MRI, cognitive neuroscientists can almost read people's thoughts from the blood flow in their brains. They can tell, for instance, whether a person is thinking about a face or a place or whether a picture the person is looking at is of a bottle or a shoe.
And consciousness can be pushed around by physical manipulations. Electrical stimulation of the brain during surgery can cause a person to have hallucinations that are indistinguishable from reality, such as a song playing in the room or a childhood birthday party. Chemicals that affect the brain, from caffeine and alcohol to Prozac and LSD, can profoundly alter how people think, feel and see. Surgery that severs the corpus callosum, separating the two hemispheres (a treatment for epilepsy), spawns two consciousnesses within the same skull, as if the soul could be cleaved in two with a knife.
And when the physiological activity of the brain ceases, as far as anyone can tell the person's consciousness goes out of existence. Attempts to contact the souls of the dead (a pursuit of serious scientists a century ago) turned up only cheap magic tricks, and near death experiences are not the eyewitness reports of a soul parting company from the body but symptoms of oxygen starvation in the eyes and brain. In September, a team of Swiss neuroscientists reported that they could turn out-of-body experiences on and off by stimulating the part of the brain in which vision and bodily sensations converge.
Consciousness is, of course, the product of the brain. The mind is an epiphenomenon of our gray matter. The manifestations of what has been traditionally called "soul" is indistinguishable from sentience, self-awareness, capacity for ethical judgement. We know that "soul" is merely a natural phenomenon because psychotropic drugs, trauma to the brain, neurological disease, hypoxia (lack of oxygen), electrical stimulation of the cortex--all these affect this so called soul. And needless to say, to date, no one has provided testable evidence for a disembodied soul, much less interaction with such (notwithstanding the litany of anecdotes out there).
Furthermore, just as there are no souls, there is no such thing as contra-causal free will. Our thoughts, actions, behavior are all the result of natural causation; they are all determined by various physical factors. This is only to be expected. And it follows from the fact that consciousness is a product of neurological processes. Those processes are caused by various determinants including neurons, chemicals (including medication we take), electrical impulses, and environmental stimuli. What we see as our freedom to choose is not freedom without causal determinants. The "I" that knows, understands, chooses, and makes decisions is an epiphenomenon of the brain.
The "I" is not a free-floating entity transcendent of and untrammeled by the natural world. Rather, it is a fully caused and fully determined phenomenon. It is as natural as the brain of which it is a derivative, and fully subject to the laws of nature.