The Mind and the Brain (4 page)

Materialism, of course, is the belief that only the physical is ontologically valid and that, going even further, nothing that is not physical—of which mind and consciousness are the paramount examples—can even exist in the sense of being a measurable, real entity. (This approach runs into problems long before minds and consciousness enter the picture: time and space are only two of the seemingly real quantities that are difficult to subsume under the materialist umbrella.) For a sense of the inadequacy of equating what neurons do with what minds experience, consider this thought experiment, based on one first advanced by the Australian philosopher Frank Jackson. Imagine a color-blind neuroscientist who has chosen to study color vision. (Jackson called her Mary.) She maps, with great precision, exactly what happens when light of a wavelength of 650 nanometers falls on the eyes of a volunteer. She
laboriously traces the pathway that analyzes color through the lateral geniculate body of the thalamus, along the sweeping fibers of the optic radiation, into the primary visual cortex. Then she carefully notes the activation of the relevant areas of the visual association cortex in the temporal lobe. The volunteer reports the outcome: he sees red! So far, so good. The neuroscientist has precisely described the stimulus—light of a precise wavelength. She has meticulously traced the brain circuits that are activated by this stimulus. And she has been told, by her volunteer, that the whole sequence adds up to a perception of red.

Can we now say that our neuroscientist knows, truly and deeply
knows
, the feeling of seeing red? She certainly knows the input, and she knows its neural correlates. But if she got out of bed the next morning to find that her color blindness had miraculously remitted, and her gaze fell on a field of crimson poppies, the experience of “red” at that instant would be dramatically and qualitatively different from the knowledge she had gained in the lab about how the brain registers the color red. Mary would now have the conscious, subjective, felt experience of color.

We needn’t belabor the point that there is a very real difference between understanding the physiological mechanisms of perception and having a conscious perceptual experience. For now, let’s say the latter has something to do with awareness of, and attention to, what is delivered for inspection by the perceptual machinery of the central nervous system. This conscious experience, this mental state called a sense of red, is not coherently described, much less entirely explained, by mapping corresponding neural activity. Neuroscientists have successfully identified the neural correlates of pain, of depression, of anxiety. None of those achievements, either, amounts to a full explanation of the mental experience that neural activity underlies. The explanatory gap has never been bridged. And the inescapable reason is this: a neural state is not a mental state. The mind is not the brain, though it depends on the material brain for its existence (as far as we know). As the philosopher Colin
McGinn says, “The problem with materialism is that it tries to construct the mind out of properties that refuse to add up to mentality.”

This is not exactly the view you find expressed at the weekly tea of a university neuroscience department. For the most part, the inevitable corollary of materialism known as identity theory—which equates brain with mind and regards the sort of neuron-to-neuron firing pattern leading to the perception of color as a full explanation of our sense of red—has the field by the short hairs. The materialist position has become virtually synonymous with science, and anything nonmaterialist is imbued with a spooky sort of mysticism (cue the
Twilight Zone
theme). Yet it is a misreading of science and its history to conclude that our insights into nature have reduced everything to the material.

The advent of materialism is widely credited to Isaac Newton, who is considered the intellectual father of the view that the world is an elaborate windup clock that follows immutable laws. (Or, as Alexander Pope put it in his famous couplet, “Nature and Nature’s laws lay hid in night:/God said, ‘Let Newton be!’ and all was light.”) But that represents a misreading of Newtonian physics. It is true that, by discovering the law of gravity, and realizing that its manifestation on Earth (that famous, if apocryphal, falling apple) and its manifestation in space (tethering the Moon to Earth, and Earth and planets to the Sun) are simply different aspects of the same phenomenon, Newton in some sense largely eliminated the divine from the ongoing workings of the universe. But Newton himself did not believe in pure materialism. Although he rid his clockwork universe of the hand of God, Newton replaced it with something just as immaterial—fields of force. In contrast to the materialist doctrine, which holds that the world is a set of objects that interact through direct contact, Newton’s theory of gravity posited action at a distance. Just how, exactly, does Earth keep the Moon from flying away into space? Through gravity. And what is gravity? An ineffable force that pervades all space and is felt over essentially infinite distances. There is no connective tissue, no intervening matter between
the mutual gravitational pulls of objects separated by vast distances across a vacuum. You cannot touch a gravitational field (although you can, of course, feel its effects). Newton himself squirmed under the implications of this: “That one body may act upon another at a distance, through a vacuum, without the mediation of anything else…is to me so great an absurdity that I believe no man who has…any competent faculty of thinking can ever fall into it. Gravity must be caused by an agent…but whether this agent be material or immaterial is a question I have left to…my readers.”

This is not the view that most people associate with classical Newtonian physics. Laypeople as well as most scientists believe that science regards the world as built out of tiny bits of matter. “Yet this view is wrong,” argues Henry Stapp, a physicist at the Lawrence Berkeley National Laboratory high in the hills above Berkeley, California. At least one version of quantum theory, propounded by the Hungarian mathematician John von Neumann in the 1930s, “claims that the world is built not out of bits of matter but out of bits of knowledge—subjective, conscious knowings,” Stapp says. These ideas, however, have fallen far short of toppling the materialist worldview, which has emerged so triumphant that to suggest humbly that there might be more to mental life than action potentials zipping along axons is to risk being branded a scientific naif. Even worse, it is to be branded nonscientific. When, in 1997, I made just this suggestion over dinner to a former president of the Society for Neuroscience, he exclaimed, “Well, then you are not a scientist.” Questioning whether consciousness, emotions, thoughts, the subjective feeling of pain, and the spark of creativity arise from nothing but the electrochemical activity of large collections of neuronal circuits is a good way to get dismissed as a hopeless dualist.

Ah, that dreaded label.

The dualist position in the mind-matter debate dates back to the seventeenth-century French philosopher René Descartes (1596–1650). Although the problem of mind and matter is as old as the philosophy of the ancient Greeks, Descartes was the first modern
scientific thinker to grapple seriously with the strangeness of mind, with the fact that the mental realm seems to be of an entirely different character from the material world. His solution was simplicity itself. He posited the existence of two parallel yet separate domains of reality:
res cogitans
, the thinking substance of the subjective mind whose essence is thought, and
res extensa
, or the extended substance of the material world. Mental stuff and material (including brain) stuff are absolutely distinct, he argued. Material substance occupies space (Descartes was big on space: he invented analytic, or Cartesian, geometry), and its behavior can be explained by one piece of matter’s mechanically pushing around another piece of matter. Descartes believed that all living things, including all “brute animals,” were just “automata or moving machines” that act “according to the disposition of their organs, just as a clock, which is only composed of wheels and weights, is able to tell the hours and measure the time more correctly than we can do with all our wisdom.” In Descartes’s mechanical clockwork cosmology, all bodies, including living bodies, were automatons, moving around like the mechanical puppets that were fashionable showpieces in the gardens of noblemen of the day. The human body was no exception. Descartes regarded the brain as a machine, subject to mechanistic, deterministic rules, and the body as an automaton. In his 1664
Traite de l’homme
, Descartes included a charming illustration modeling reflexive behavior. He showed a man’s foot edging into a fire; the message “hot!” is depicted traveling through sensory nerves to the head and then back down to a muscle in the leg. This path results in the foot’s reflexively pulling out of the blaze. Descartes’s careful tracing of the path is one of the earliest examples of those endless neural-correlates discoveries so beloved of twentieth-century neuroscientists.

Descartes defined mind, in contrast to matter, by what it lacks—namely, spatial extent and heft. And he recognized another difference. Reflexes and other attributes or expressions of matter, he argued, are subject to scientific inquiry. Conscious, subjective expe
rience is not. Descartes’s separation of nature into a physical realm and a mental/experiential realm, each dynamically independent of the other, thus gave an indirect benefit to science. The seventeenth century saw what threatened to be a to-the-death struggle between science and the Church, which perceived science as a threat. Descartes’s declaration that reality divides neatly into two realms reassured the Church that the province of science would never overlap, and therefore never challenge, the world of theology and the spiritual. Science ceded the soul and the conscious mind to religion and kept the material world for itself. In return for this neat dividing up of turf, Descartes hoped, religious leaders would lay off scientists who were studying natural laws operating in the physical, nonmental realm. The ploy was only partly successful for Church-science relations. Descartes himself was forced to flee Paris for Holland in search of greater tolerance.

But this division of reality into mind and matter was also something of a scientific debacle. Separating the material and the mental into ontologically distinct realms raised the white flag early in the mind-body debate: science abandoned the challenge of explaining how the components of the physical world found expression in the mental world. And thus was Cartesian dualism born. Today, three and a half centuries later, his belief endures. If there is a single fundamental underpinning in the intellectual tradition of Western scientific thought, it is arguably that there exists an unbridgeable divide between the world of mind and the world of matter, between the realm of the material (which is definitely real) and the realm of the immaterial (which, according to the conventions of science, is likely illusory).

Yet Cartesian dualism ran into trouble almost immediately. Descartes’s material automaton was, in its human form, an automaton with a difference: it was capable of voluntary, volitional, freely willed movement. By exerting its will, Descartes declared, the immaterial human mind could cause the material human machine to move. This bears repeating, for it is an idea that, more
than any other, has thrown a stumbling block across the path of philosophers who have attempted to argue that the mind is immaterial: for how could something immaterial act efficaciously on something as fully tangible as a body? Immaterial mental substance is so ontologically different—that is, such a different sort of thing—from the body it affects that getting the twain to meet has been exceedingly difficult. To be sure, Descartes tried. He argued that the mental substance of the mind interacts with the matter of the brain through the pineal gland, the organ he believed was moved directly by the human soul. The interaction allowed the material brain to be physically directed by the immaterial mind through what Descartes called “animal spirits”—basically a kind of hydraulic fluid.

Even in his own time Descartes’s dualism fell far short of carrying the day, and its principal antagonist, materialism, quickly reared its head. In the mid-1600s, with the advent of neuroscience, researchers began to piece together new theories of the relationship between mind and brain, discovering basic biological mechanisms underlying conscious feelings and thoughts. On the basis of these findings, the French physician Julien Offray de la Mettrie (1709–1751) asserted that mind and brain are merely two aspects of the same physical entity, and that this three-pound collection of cells sitting inside our skull either entirely determines, or is somehow identical with, mental experience. In his 1747 book
L’homme machine
(Man the Machine), La Mettrie gained notoriety by attempting to show that humans are in essence nothing but automatons. In this he was taking to its logical conclusion a chain of reasoning that had begun when Descartes proclaimed an entirely mechanical understanding of every living thing
save
humans. Even more than Descartes, La Mettrie applied the methods of experimental medical science to bolster his bold claim. He described the brain as the organ of thinking and maintained that brain size determines mental capacity. And he compared the workings of the brain to those of musical instruments. “As a violin string or a key of the
clavichord vibrates and renders a sound,” he wrote, “so the brain’s chords struck by sound waves are stimulated to render or to repeat the words which touch it.” Perhaps the most remarkable aspect of La Mettrie’s perspective is how contemporary it sounds in this, the age of computer intelligence.

Thus were born the dueling ontologies, with partisans of matter like La Mettrie squaring off against those like Descartes who believed that mental events cannot all be reduced to physical ones. For more than three centuries after Descartes published his thesis, philosophers battled over which entity, mind or matter, was the basic stuff of the world. Philosophers including Leibniz, Berkeley, Hume, Kant, Mach, and James contended that matter is but a uniquely objective and substantial form of mind. This position is not far different from that held by many contemporary physicists, who believe that matter is merely a concentrated form of energy. It is this position that most closely mirrors my own. On the other side of the dualist divide, thinkers such as Hobbes, La Mettrie, Marx, Watson, B.F. Skinner, and Daniel Dennett have argued what has become the consensus position of mainstream science: that mind truly is, in essence, nothing but matter, and our subjective experience that mind is something special or different is just an illusion. The mind is entirely and completely derived from the brain’s matter.