The complexity of all this far exceeds the amount of design information your genes can carry, so in effect you design and build your own brain.

Each brain is unique, formed by its own upbringing and experiences. Some of these developments are deliberate - if you decide to learn the piano, for example, or your mother teaches you to read - but most of them just happen. So this is unavoidably a rather mysterious implement.

Until recently you could not even look at one unless it had first been switched off, with fatal consequences for the user. At the same time the brain is of compelling interest. We all know, think and feel.

Many people throughout the ages have often wondered precisely what was involved in these activities.

Traditionally there have been two approaches to the problem. The 'wet' approach involves cutting up brains or looking closely at people whose brains have already been battered by some accident or illness.

The 'dry' approach is more cerebral and involves examining outputs - what can we tell about thought from the way it is expressed - and introspection. Both approaches have made considerable progress recently and in this book Damasio tries to bring them together. Psychology, physiology and philosophy converge with - necessarily - a certain amount of speculation bridging the gaps.

His basic idea, which is festooned with daunting technicalities but intrinsically fascinating, is that your brain is not an isolated thinking machine stuck on top of your body, with a few tubes to keep it supplied with oxygen.

On the contrary, thinking is a process which involves a constant interaction with the rest of the body.

Your gut instincts do indeed actually involve your guts. The constant internal dialogue between body and brain is what keeps your brain alert and interested.

Information and influences flow in both directions. When you feel happy you smile. When you smile you feel happy (a platitude but literally true). This line of thought has implications for many people. Think again, those science fiction writers who have disembodied brains sitting in vats of nutrients, running planets and exchanging philosophical profundities with the visiting crew of the starship Enterprise.

There is actually an ailment which produces this effect, cutting the brain off from all signals from the rest of the body. Patients swiftly lapse into a state of profound lethargy and lack of emotion. They cease, in effect, to be recognisable as people. Those companies who market a service in which your head is cut off, post mortem, to be frozen and revived when medical science has advanced far enough to give you a new lease on life, are either working on a very long time scale or perpetrating a gross fraud.

And poor Descartes, who started his Principles of Philosophy with 'I think, therefore I am,' got it wrong from the beginning.

Existence precedes thought, conditions it, motivates it. Your brain's first job was to run your body and that is still its main preoccupation. Everything else is a luxury.

This is not always an easy book to read. Damasio is a Professor of Neurology and sometimes assumes a lot. His publishers would have done readers a great service if they had included an inflatable model of the brain, suitably labelled, for those of us who think the hippocampus is the place where St Augustine went to university.

The tone is by no means clinical, though. There are encounters with Tolstoy, Pascal and Wagner as well as more predictable landmarks like Henry James and Freud.

We also meet some nameless but interesting individuals from Damasio's clinical experience, with a variety of curious limitations on their brain activity. Incredible but true. Looking at what goes wrong when something is broken was the traditional way of discovering which parts of the brain do what, and with modern imaging techniques this has become a goldmine.

The refreshing thing about Damasio's approach, though, is that it is not clinical. He has managed to avoid the new-look Darwinian reductionism.

Here is a man who understands all the currently fashionable theories about evolution, who knows what a neuron looks like and as much about how a brain really works as anyone does.

Yet this does not in the least diminish his appreciation of the essential mysteries.

A detailed analysis of the mechanisms involved in emotion ends with this warning: 'It is important to realise that defining emotion and feeling as concrete, cognitively and neurally, does not diminish their loveliness or horror, or their status in poetry or music.

'Understanding how we see or speak does not debase what is seen or spoken, what is painted or woven into a theatrical line.

'Understanding the biological mechanisms behind emotions and feelings is perfectly compatible with a romantic view of their value to human beings.' On this understanding, Damasio is welcome to poke about inside my head any time he likes.