STEVEN PINKER: I'm going to discuss an idea that elicits wildly opposite reactions. Some people find it a shocking claim with radical implications for morals and every value that we hold dear. Other people think that it's a claim that was established a hundred years ago, that the excitement is only in how we work out the details, and that it has few if any implications for our values and ethics. That is the idea that the mind is the physiological activity of the brain, in particular the information processing activity of the brain; that the brain, like other organs, is shaped by the genes; and that in turn, the genome was shaped by natural selection and other evolutionary processes. I am among those who think that this should no longer be a shocking claim, and that the excitement is in fleshing out the details, and showing exactly how our perception, decision-making, and emotions can be tied to the activity of the brain.

Three new sciences are now vividly rooting our mental processes in our biology. Cognitive neuroscience, the attempt to relate thought, perception and emotion to the functioning of the brain, has pretty much killed Soul One, in Richard's sense. It should now be clear to any scientifically literate person that we don't have any need for a ghost in the machine, as Gilbert Ryle memorably put it. Many kinds of evidence show that the mind is an entity in the physical world, part of a causal chain of physical events. If you send an electric current through the brain, you cause the person to have a vivid experience. If a part of the brain dies because of a blood clot or a burst artery or a bullet wound, a part of the person is gone -- the person may lose an ability to see, think, or feel in a certain way, and the entire personality may change. The same thing happens gradually when the brain accumulates a protein called beta-amyloid in the tragic disease known as Alzheimer's. The person -- the soul, if you want -- gradually disappears as the brain decays from this physical process.

We know that every form of mental activity -- every emotion, every thought, every percept -- gives off electrical, magnetic, or metabolic signals that can be recorded with increasing precision by Positron Emission Tomography, functional Magnetic Resonance Imaging, Magnetoencephalography, and other techniques. We know that if you take a knife and section the corpus callosum (which joins the two cerebral hemispheres) you have the equivalent of two minds -- perhaps even two souls -- in the same skull. We know that if you look at the brain under a microscope it has a breathtaking degree of complexity -- on the order of a trillion synapses -- that's fully commensurate with the breathtaking complexity of human thought and experience. We know that when the brain dies, the person goes out of existence. I consider it to be a significant empirical discovery that one cannot communicate with the dead, and excellent evidence that Soul One, in Richard's sense, does not exist.

A second science, behavioral genetics, has shown that there is a fascinating degree of specificity in our genome. You've all heard of the remarkable studies of monozygotic twins reared apart, who are remarkably similar in intelligence, personality, and attitudes -- even in their opinion on the death penalty and their tastes in music and clothing. And just in the past year there have been discoveries of genetic markers, and in some case genes and even gene products, associated with mental traits such as intelligence, spatial cognition, control of speech, the desire to seek sensation, and the tendency to be overly anxious.

The third science that's connecting mind to biology is evolutionary psychology, which takes an approach to understanding the mind that has long been fruitful in understanding the organs of the body. We can't make sense of an organ like the eye without considering it to have a function, or a purpose - not in a mystical, teleological sense, but in the sense of an illusion of engineering. That illusion, we now know, is a consequence of Darwin's process of natural selection. Everyone agrees that the eye is a remarkable bit of natural "engineering," and that may now be explained as a product of natural selection rather than as the handiwork of a cosmic eye-designer or as a massive coincidence in tissue formation. But the eye by itself is useless -- unless it's connected to a brain. The eye does not carry out its function by dumping optical information into a yawning chasm. Rather, the eye is hooked up to parts of the brain -- anatomically speaking, the eye is an extension of the brain -- and those parts contain circuits for analyzing the incoming visual material, for recovering the shapes and colors and motions in the world that gave rise to the stimulation of the eye. The perception of a world of colored 3-D objects, in turn, feeds into a system of categorization, allowing us to make sense of our experience, to impute causes to events, and to remember things in terms of their significant categories. And in turn, those categories themselves would be useless unless they were organized in service of certain goals, goals set by our emotions. Beginning with the eye, we have a chain of causation that leads to the study of faculties of mind, or modules, or subsystems, each of which can be seen as an adaptation akin to the adaptations in the organs of the body. Recent research has shown that aspects of the psyche that were previously considered mysterious, quirky, and idiosyncratic -- such as phobias, an eye for beauty, the tendency to fall in love, a passionate desire for revenge in defense of honor -- turn out to have a subtle evolutionary logic when they are analyzed in the way in which we have always analyzed the organs of the body.