Read an Excerpt

Chapter 1

Introduction
Q: What is the soul?

A: The soul is a living being without a body, having reason and free will.

Roman Catholic catechism

The Astonishing Hypothesis is that "You," your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules. As Lewis Carroll's Alice might have phrased it: "You're nothing but a pack of neurons." This hypothesis is so alien to the ideas of most people alive today that it can truly be called astonishing.

The interest of human beings in the nature of the world, and about their own natures in particular, is found in one form or another in all peoples and tribes, however primitive. It goes back to the earliest times from which we have written records and almost certainly from before that, to judge from the widespread occurrence of careful human burial. Most religions hold that some kind of spirit exists that persists after one's bodily death and, to some degree, embodies the essence of that human being. Without its spirit a body cannot function normally, if at all. When a person dies his soul leaves the body, although what happens after that — whether the soul goes to heaven, hell, or purgatory or alternatively is reincarnated in a donkey or a mosquito — depends upon the particular religion. Not all the religions agree in detail, but this is usually because they are based on different revelations — contrast the Christian Bible and the Muslim Koran. In spite of differences among religions, there is broad agreement on at least one point: People have souls, in the literal and not merely the metaphorical sense. These beliefs are held, and in many cases held strongly and aggressively, by the majority of human beings alive today.

There are, of course, a few exceptions. At one point, a minority of some of the more extreme Christians (following Aristotle) doubted whether women had souls, or at least had souls of the same quality as men. Some religions, such as Judaism, put little emphasis on life after death. Religions differ as to whether animals have souls. An old joke suggests that philosophers (in spite of all their differences) fall broadly into two classes: Those who own dogs, it is said, are confident that dogs have souls; those who do not, deny this.

Yet a minority of people today (including a large number in the former Communist countries) is inclined to a totally different view. Such people believe that the idea of a soul, distinct from the body and not subject to our known scientific laws, is a myth. It is easy to see how such myths could have arisen. Indeed, without a detailed knowledge of the nature of matter and radiation, and of biological evolution, such myths appear only too plausible.

Why, then, should this basic concept of the soul be doubted? Surely if almost everyone believed it, this is, in itself, prima facie evidence for it. But then some four thousand years ago almost everyone believed the earth was fiat. The main reason for this radical change of opinion is the spectacular advance of modern science. Most of the religious beliefs we have today originated in a time when the earth, while a small place by our standards, was then thought of as being very large, even though its exact extent was unknown. Any one person had direct knowledge of only a tiny part of it. It was not implausible to believe that this large earth was the center of the universe and that man occupied the leading place in it. The earth's origins seemed lost in the mists of time and yet the span of time thought to be involved, while it seemed long in terms of human experience, we now know to be ridiculously short. It was not implausible to believe the earth was less than ten thousand years old. We now know its true age is about 4.6 billion years. The stars seemed far away, fixed perhaps in the spherical firmament, but that the universe extended as far as it does more than 10 billion light years — was almost inconceivable. (An exception has to be made here for certain Eastern religions, such as Hinduism, that take pleasure in inflating times and distances for the sheer joy of it.)

Before Galileo and Newton our knowledge of basic physics was primitive. The movements of the sun and the planets seemed to be regular in some very intricate way. It was not totally unreasonable to believe that they needed angels to guide them. What else could make their behavior so regular? Even in the sixteenth and seventeenth centuries our understanding of chemistry was largely incorrect. Indeed, the existence of atoms was doubted by some physicists as late as the beginning of the twentieth century.

Yet today we know a great deal about the properties of atoms. We can give each type of chemical atom a unique whole number. We know their structure in detail, and most of the laws that control their behavior. Physics has provided a framework of explanation for chemistry. Our detailed knowledge of organic chemical molecules is enormous and expanding every day.

Admittedly, we still do not understand exactly what happens at very small distances (within the nucleus of an atom), at extremely high energies, and in very large gravitational fields. But for the conditions we normally have to deal with on earth (when an atom will only change into another atom under very special circumstances), most scientists think that this incompleteness of our knowledge probably matters very little, if at all, in our attempts to understand the mind and the brain.

In addition to our knowledge of basic chemistry and physics, the earth sciences (such as geology) and cosmic science (astronomy and cosmology) have developed pictures of our world and our universe that are quite different from those common when the traditional religions were founded. The modern picture of the universe, and how it developed in time, forms an essential background to our present knowledge of biology. That knowledge has been completely transformed in the last 150 years. Until Charles Darwin and Alfred Wallace independently hit on the basic mechanism driving biological evolution — the process of natural selection — the "Argument from Design" appeared to be unanswerable. How could an organism as complex and well designed as man have arisen without the help of an all-wise Designer? Yet this argument has collapsed completely. We now know that all living things, from bacteria to ourselves, are closely related at the biochemical level. We know that life has existed on earth for billions of years, and that the many species of plants and animals have changed, often radically, over that time. The dinosaurs have gone and in their place many new species of mammals have arisen. We can watch the basic processes of evolution happening today, both in the field and in our test tubes.

In this century there has been an equally dramatic biological advance, due to our understanding of the molecular nature of genes, the processes involved in their exact replication, together with our detailed knowledge of proteins and the mechanisms of their synthesis. We now realize that proteins, as a class, are immensely powerful and versatile, and can form the basis of elaborate biochemical devices. A major assault is being mounted on embryology (now often called "developmental biology"). The fertilized egg of a sea urchin normally divides many times, eventually turning into a mature sea urchin. If, after the first division, the two daughter cells of the fertilized egg are separated, then each of them will develop, in time, into a separate though rather smaller sea urchin. A similar experiment can be done on a frog's egg. The molecules have reorganized themselves to produce two little animals from the material that would otherwise have produced one. When this was first discovered, about a hundred years ago, it was suggested that some kind of immaterial Life Force must surely be at work. It seemed inconceivable that this dramatic doubling of a living creature could ever be explained on a biochemical basis — that is, using the properties of organic and other molecules and their interactions. Nowadays we feel that we shall have no difficulty in principle in working out how this can happen, although we expect the explanation to be complex. The history of science is littered with statements that something was inherently impossible to understand ("we shall never know of what the stars are made"). In many cases time has shown these predictions to be incorrect.

A modern neurobiologist sees no need for the religious concept of a soul to explain the behavior of humans and other animals. One is reminded of the question Napoleon asked after Pierre-Simon Laplace had explained to him the workings of the solar system: Where does God come into all this? To which Laplace replied, "Sire, I have no need of that hypothesis." Not all neuroscientists believe that the idea of the soul is a myth — Sir John Eccles is the most notable exception — but certainly the majority do. It is not that they can yet prove the idea to be false. Rather, as things stand at the moment, they see no need for that hypothesis. Looked at in the perspective of human history, the main object of scientific research on the brain is not merely to understand and cure various medical conditions, important though this task may be, but to grasp the true nature of the human soul. Whether this term is metaphorical or literal is exactly what we are trying to discover.

Many educated people, especially in the Western world, also share the belief that the soul is a metaphor and that there is no personal life either before conception or after death. They may call themselves atheists, agnostics, humanists, or just lapsed believers, but they all deny the major claims of the traditional religions. Yet this does not mean that they normally think of themselves in a radically different way. The old habits of thought die hard. A man may, in religious terms, be an unbeliever but psychologically he may continue to think of himself in much the same way as a believer does, at least for everyday matters.

We need, therefore, to state the idea in stronger terms. The scientific belief is that our minds — the behavior of our brains — can be explained by the interactions of nerve cells (and other cells) and the molecules associated with them. This is to most people a really surprising concept. It does not come easily to believe that I am the detailed behavior of a set of nerve cells, however many there may be and however intricate their interactions. Try for a moment to imagine this point of view. ("Whatever he may say, Mabel, I know I'm in there somewhere, looking out on the world.")

Why does the Astonishing Hypothesis seem so surprising? I think there are three main reasons. The first is that many people are reluctant to accept what is often called the "reductionist approach" — that a complex system can be explained by the behavior of its parts and their interactions with each other. For a system with many levels of activity, this process may have to be repeated more than once — that is, the behavior of a particular part may have to be explained by the properties of its parts and their interactions. For example, to understand the brain we may need to know the many interactions of nerve cells with each other; in addition, the behavior of each nerve cell may need explanation in terms of the ions and molecules of which it is composed.

Where does this process end? Fortunately there is a natural stopping point. This is at the level of the chemical atoms. Each atom consists of a heavy atomic nucleus, carrying a positive charge, surrounded by an organized cloud of light, negatively charged nimble electrons. The chemical properties of each atom are determined almost entirely by its nuclear charge. The other properties of the nucleus — its mass, its secondary electrical properties such as the strengths of its dipole, and its quadrupole — make in most cases only small differences to its chemical properties.

Now, to a first approximation, the mass and charge of the nucleus of an atom never change, at least in the mild environment in which life flourishes on earth. Thus the knowledge of the substructure of the nucleus is not needed for chemistry. It makes no difference that an atomic nucleus is composed of various combinations of protons and neutrons, and that they, in turn, are made up of quarks. All the chemist needs to know about each atom is its nuclear charge in order to explain most of the facts of chemistry. To do this he needs to understand the rather unexpected type of mechanics (called "quantum mechanics") that controls the behavior of very small particles and of electrons in particular. In practice, since the calculations soon become impossibly intricate, he mainly uses various rules-of-thumb that we now can see have a reasonable explanation in terms of quantum mechanics. Below this level he need not venture.

There have been a number of attempts to show that reductionism cannot work. They usually take the form of a rather formal definition, followed by an argument that reductionism of this type cannot be true. What is ignored is that reductionism is not the rigid process of explaining one fixed set of ideas in terms of another fixed set of ideas at a lower level, but a dynamic interactive process that modifies the concepts at both levels as knowledge develops. After all, "reductionism" is the main theoretical method that has driven the development of physics, chemistry, and molecular biology. It is largely responsible for the spectacular developments of modern science. It is the only sensible way to proceed until and unless we are confronted with strong experimental evidence that demands we modify our attitude. General philosophical arguments against reductionism will not do.

Another favorite philosophical argument is that reductionism involves a "category mistake." For example, in the 1920s this could have taken the form that to consider a gene to be a molecule (or as we should say now, part of a matched pair of molecules) would be a category mistake. A gene is one category and a molecule is a quite different category. One can see now how hollow such objections have turned out to be. Categories are not given to us as absolutes. They are human inventions. History has shown that although a category may sound very plausible, it can, on occasion, turn out to be both misconceived and misleading. Recall the four Humors in ancient and medieval medicine: blood, phlegm, choler, and black bile.

The second reason why the Astonishing Hypothesis seems so strange is the nature of consciousness. We have, for example, a vivid internal picture of the external world. It might seem a category mistake to believe this is merely another way of talking about the behavior of neurons, but we have just seen that arguments of this type are not always to be trusted.

Philosophers have been especially concerned with the problem of qualia — for example, how to explain the redness of red or the painfulness of pain. This is a very thorny issue. The problem springs from the fact that the redness of red that I perceive so vividly cannot be precisely communicated to another human being, at least in the ordinary course of events. If you cannot describe the properties of a thing unambiguously, you are likely to have some difficulty trying to explain those properties in reductionist terms. This does not mean that, in the fullness of time, it may not be possible to explain to you the neural correlate of your seeing red. In other words, we may be able to say that you perceive red if and only if certain neurons (and/or molecules) in your head behave in a certain way. This may, or may not, suggest why you experience the vivid sensation of color and why one sort of neural behavior necessarily makes you see red while another makes you see blue, rather than vice versa.

Even if it turns out that the redness of red cannot be explained (because you cannot communicate that redness to me), it does not follow that we cannot be reasonably sure that you see red in the same way as I see red. If it turns out that the neural correlate of red is exactly the same in your brain as in mine, it would be scientifically plausible to infer that you see red as I do. The problem lies in the word "exactly." How precise we have to be will depend on a detailed knowledge of the processes involved. If the neural correlate of red depends, in an important way, on my past experience, and if my past experience is significantly different from yours, then we may not be able to deduce that we both see red in exactly the same way.

One may conclude, then, that to understand the various forms of consciousness we first need to know their neural correlates.

The third reason why the Astonishing Hypothesis seems strange springs from our undeniable feeling that our Will is free. Two problems immediately arise: Can we find a neural correlate of events we consider to show the free exercise of our Will? And could it not be that our Will only appears to be free? I believe that if we first solve the problem of awareness (or consciousness), the explanation of Free Will is likely to be easier to solve. (This topic is discussed at greater length in the Postscript on page 265.)

How did this extraordinary neuronal machine arise? To understand the brain, it is important to grasp that it is the end product of a long process of evolution by natural selection. It has not been designed by an engineer, even though, as we shall see, it does a fantastic job in a small space and uses relatively little energy to do it. The genes we received from our parents have, over many millions of years, been influenced by the experience of our distant ancestors. These genes, and the processes directed by them before birth, lay down much of the structure of the parts of our brain. The brain at birth, we now know, is not a tabula rasa but an elaborate structure with many of its parts already in place. Experience then tunes this rough-and-ready apparatus until it can do a precision job.

Evolution is not a clean designer. Indeed, as Francois Jacob, the French molecular biologist, has written. "Evolution is a tinkerer." It builds, mainly in a series of smallish steps, on what was there before. It is opportunistic. If a new device works, in however odd a manner, evolution will try to promote it. This means that changes and improvements that can be added to the existing structures with relative ease are more likely to be selected, so the final design may not be a clean one but rather a messy accumulation of interacting gadgets. Surprisingly, such a system often works better than a more straightforward mechanism that is designed to do the job in a more direct manner.

Thus the mature brain is the product of both Nature and Nurture. We can see this easily in the case of language. The ability to handle a complex language fluently appears to be unique to human beings. Our nearest relatives, the apes, perform very poorly at language use even after extensive training. Yet the actual language we learn is obviously heavily dependent on where and how we were brought up.

Two more philosophical points need to be made. The first is that much of the behavior of the brain is "emergent" — that is, behavior does not exist in its separate parts, such as the individual neurons. An individual neuron is in fact rather dumb. It is the intricate interaction of many of them together that can do such marvelous things.

There are two meanings of the term emergent. The first has mystical overtones. It implies that the emergent behavior cannot in any way, even in principle, be understood as the combined behavior of its separate parts. I find it difficult to relate to this type of thinking. The scientific meaning of emergent, or at least the one I use, assumes that, while the whole may not be the simple sum of the separate parts, its behavior can, at least in principle, be understood from the nature and behavior of its parts plus the knowledge of how all these parts interact.

A simple example, from elementary chemistry, would be any organic compound, such as benzene. A benzene molecule is made of six carbon atoms arranged symmetrically in a ring with a hydrogen atom attached, on the outside of the ring, to each carbon atom. Apart from its mass, the properties of a benzene molecule are not in any sense the simple arithmetical sum of the properties of its twelve constituent atoms. Nevertheless, the behavior of benzene, such as its chemical reactivity and its absorption of light, can be calculated if we know how these parts interact, although we need quantum mechanics to tell us how to do this. It is curious that nobody derives some kind of mystical satisfaction by saying "the benzene molecule is more than the sum of its parts," whereas too many people are happy to make such statements about the brain and nod their heads wisely as they do so. The brain is so complicated, and each brain is so individual, that we may never be able to obtain second-to-second detailed knowledge of how a particular brain works but we may hope at least to understand the general principles of how complex sensations and behaviors arise in the brain from the interactions of its many parts.

Of course there may be important processes going on that have not yet been discovered. I suspect that even if we were told the exact behavior of one part of the brain we might in some cases not immediately understand the explanation, since it might involve new concepts and new ideas that have yet to be articulated. However, I do not share the pessimism of some who think that our brains are inherently incapable of grasping such ideas. I prefer to confront such difficulties, if indeed they exist, when we come to them. Our brains have evolved and developed so that we can deal fluently with many concepts related to our everyday world. Nevertheless, well-trained brains can grasp ideas about phenomena that are not part of our normal experience, such as those of relativity and quantum mechanics. Such ideas are very counterintuitive, but constant practice with them enables the trained brain to grasp them and manipulate them easily. Ideas about our brains are likely to have the same general character. They may appear very strange at first but with practice we may hope to handle them with confidence.

There is no obvious reason why we should not be able to obtain this knowledge — both of the components of the brain and also how they interact together. It is the sheer variety and complexity of the processes involved that makes our progress so slow.

The second philosophical conundrum that needs clarification concerns the reality of the outside world. Our brains have evolved mainly to deal with our body and its interactions with the world it senses to be around us. Is this world real? This is a venerable philosophical issue and I do not wish to be embroiled in the finely honed squabbles to which it has led. I merely state my own working hypotheses: that there is indeed an outside world, and that it is largely independent of our observing it. We can never fully know this outside world, but we can obtain approximate information about some aspects of its properties by using our senses and the operations of our brain. Nor, as we shall see, are we aware of everything that goes on in our brains, but only of some aspects of that activity. Moreover, both these processes — our interpretations of the nature of the outside world and of our own introspections — are open to error. We may think we know our motives for a particular action, but it is easy to show that, in some cases at least, we are in fact deceiving ourselves.

Copyright © 1994 by The Francis H. C. Crick and Odile Crick Revocable Trust