BOOK REVIEW / Chemistry and an attack of nerves: 'The Astonishing Hypothesis: The Scientific Search for the Soul' - Francis Crick: Simon & Schuster, 16.99

IN 1953 Francis Crick changed the face of science. Together with James Watson he cracked the genetic code, the chemical structure of DNA. Since then molecular biology has piled advance upon spectacular advance, and we now stand on the threshold of an age of genetic engineering.

Crick and Watson made a good team. Where the American Watson was an ambitious young career-scientist, Crick was an ideas man. A physicist who strayed into biology, he had been a young boffin during the war. By the time Watson met him, he already had an enviable reputation for penetrating to the heart of any scientific problem. The only trouble was that these were often his colleagues' problems, and some of them were starting to ask whether he would ever stick at anything long enough to make a contribution.

The structure of DNA provided their answer. Crick continued to work on the chemistry of genes for some years after this initial breakthrough. But eventually he felt that the basic problems had been solved, and in 1976, at the age of 60, he decided to switch subjects once more. Since then he has been working on the neurophysiology of the brain and the nature of human consciousness.

The Astonishing Hypothesis reports on this work. It is a fascinating book, even if it is ultimately unpersuasive. Crick brings a physicist's perspective to the problems of mind. He thinks that the key to consciousness lies in the workings of our nerve cells, and that only a detailed programme of experimentation can uncover its secrets.

Crick displays a physicist's impatience with the looser approaches to mind found among most philosophers and psychologists. This insistence on hard theorising served him well as a molecular biologist, but there are two reasons why it hampers him as a theorist of consciousness. One is that we need a 'top-down' understanding of the mind as much as a 'bottom-up' one. We need to understand the general structure of our cognitive abilities, not just the chemistry of our neurotransmitters. Trying to work out what the brain is doing by looking at nerve cells is like trying to work out what a poem says by counting the letters in it.

This is particularly clear with visual awareness, the topic Crick concentrates on. We now have an extremely detailed understanding of the neurones in the visual cortex, or at least in the visual cortex of macaque monkeys. More than 30 areas of the cortex have been identified, each displaying specialised responses to movements, colours, lines and even faces. But how our brain puts all this information together and guides us through the world remains a mystery.

When Crick did his work on DNA, biochemists already understood the overall role of the genetic material in regulating cellular activities. Crick's problem was simply to work out which physical structure plays this role. In the same way, we first need to understand the overall part that visual awareness plays in our psychology, if we are to have any chance of identifying the neuronal mechanisms that embody it.

The other reason consciousness is unlikely to succumb to physics is that in large part it isn't a scientific problem at all. Crick's 'astonishing hypothesis' is that the mind is just the body: what we call consciousness is simply another kind of brain activity; there is no soul, or separate spirit, just the behaviour of millions of nerve cells. However, this hypothesis is not that astonishing, given that it follows from a knock-down argument familiar to generations of philosophers. This is the argument that as the mind produces physical effects, namely our bodily movements, it must be physical itself. The only problem with this argument is that it seems to leave feelings unexplained. Why should it feel like anything to be a physical system? Wouldn't our brains work just the same even if we were zombies without any sensations?

The task facing a defender of Crick's astonishing hypothesis is to show how these questions are somehow misconceived. But this is a philosophical task, not a scientific one. Unfortunately Crick is no philosopher, and his insistence that the problem of consciousness needs experimental investigation leads him down some curious pathways. It is difficult to take seriously his suggestion that visual awareness may consist of 40-Hertz oscillations in neuronal firing, or that free will is located in the anterior cingulate sulcus.

Even so, this book is well worth reading. Crick is an excellent guide to contemporary brain science, and takes his readers deeper than most popular authors. He includes intriguing material on topics from neural nets to brain damage, and the book sparkles with the many speculations he appends to established theory. Not surprisingly, Crick's foray into neurophysiology has not proved as fruitful as his years in biology, but by normal standards this book is still an impressive achievement.