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Dreaming is commonplace, inevitable, and incomprehensible. Yet if we knew more about dreams then perhaps we would also have greater insight into the nature of consciousness in general. With this quest in sight J. Allan Hobson starts off from a laudable though unoriginal standpoint that, despite the conventional dichotomy between psychiatry and neurology, there is no true distinction between the mind and the brain: the two are one, the "brain-mind". The ideas that are subsequently on offer amount not so much to the final magic bullet for understanding mental function and dysfunction, but rather are best viewed as a catalyst for debate: for it is no easy task to forge the brain-mind.

Brain cells communicate by means of a variety of chemicals, transmitters. Different transmitter systems predominate in different states of consciousness: amines during wakefulness and acetylcholine during dreaming. From here Hobson goes on to express the normal and abnormal processes occurring in the brain-mind almost exclusively in terms of the actions of these two major chemical systems. However, a transmitter is merely a small, banal molecule: in itself it contains no magic property. The secret of the power of transmitters lies in their interaction with a specialised dock on a target brain cell. There are varieties of docks for any one transmitter, enabling it to have a complex repertoire of possible effects upon the electrical signalling of the subsequent neuron. It is therefore hard to see how we can understand how the amines and acetylcholine each play a crucial part in different states of consciousness, until we explore more thoroughly whereabouts they are each acting and what kind of effects they are having on their respective cellular targets. The full profile of such transmitter action at the level of the brain could then be related tothe mind, the shifting from wakefulness to dreaming.

As it stands, however, the book's nonspecialist reader is in danger of assuming, erroneously, that chemicals per se have self-contained functions locked into their molecular structure.

Another possible misconception the unwary reader might acquire is that the brain is a virtual chemical cauldron of mind-bending substances potentially at large throughout all parts of the brain. Obviously no one wants an unnecessary lesson in neuroanatomy, but some working overview of how the brain is put together might have done true justice to the central tenet of the brain-mind in showing the site-selectivity of different transmitters. Unfortunately, if brain regions are mentioned at all they are given coy labels like "our spatial memory bank" or "our emotion register": again the general reader might be misled, this time into thinking that each region is some kind of autonomous mini-brain.

A further conceptually unpalatable feature that is introduced at several stages is the idea of "top-down" versus "bottom-up" control of the brain-mind. Top-down control by the outer layer of the brain, the cortex, is viewed as voluntary where "you" are in charge: bottom-up control on the other hand is where "you" are no longer in control, such as when dreaming or if schizophrenic. But both sets of chemicals, the amines and acetylcholine, originate from the same, more primitive, brain region, and both influence signals sent up to the cortex. So, why should one chemical allow "you" to have your say but the other not? And can we really think of the brain as split into the voluntary "you" part and the "non-you" region? This is perilously close to the now discredited idea of a centre for consciousness, and surely against the very tenet of the brain-mind.

Hobson pays little attention to individual substances by lumping the amines together as a single and cohesive chemical system. One amine in particular, dopamine, is presumably suppressed, along with the others, during dreaming. If, as Hobson asserts, dreaming were similar to schizophrenia, and if both were simply due to the prevailing chemical system, then we would expect low dopamine levels to be associated with schizophrenia. In fact current knowledge points in the opposite direction: schizophrenia is treated by drugs that actually antagonise dopamine, while drugs raise levels of brain dopamine lead to symptoms resembling those in schizophrenia. This anomaly does not invalidate the interesting and reasonable suggestion that schizophrenia and dreaming are similar states of mind, but rather shows that neither state can be explained solely in terms of a chemical in the brain. We need to know where the relevant chemicals are acting, and what they are doing in each case, to discover the true physical basis of the parallel between our nightly experiences and a psychosis.

But what, after all, is the function of dreams? Hobson suggests that dreams might be to enforce memory. The objection here would be that memory formation probably involves changes in protein synthesis, but protein synthesis in the brain during dreaming is identical to wakefulness. A later idea is that dreams are a rehearsal for procedures that will be engaged in when subsequently awake. We read that the foetus at one stage spends its entire time dreaming. But how might a foetus know what procedure it will need when born? The whole point about the human brain is that although it is the size of chimpanzee's at birth, unlike the chimpanzee, it continues to grow in early life in a way adaptable to its particular environment. There would be no point rehearsing a procedure, when it was still unknown to you as a foetus whether you were to survive on the savannah of 30,000 years ago or eventually to master a computer game.

A further hypothesis is that dreams enable us to rehearse emotions: however Hobson later admits that we cannot remember most of our dreams, so it would be hard to envisage how such rehearsal might thus have been beneficial. Finally, we end up with the idea that dreaming simply enables the replenishment of the chemical system (amines) that are suppressed by a now active, antagonistic (acetylcholine) system. It is hard to see why replenishment of chemicals could not occur in nondreaming sleep, where indeed synthesis of proteins is significantly higher then in either dreaming or wakefulness.

Leaving aside the secrets of consciousness and dreams, this book is a rich source of information on mental functions that the general reader would have no problems in following. Perhaps most important of all is the cogent case against dismissing the action of placebos and of explaining fears and anxieties as "only psychological". By focusing on the idea of the physical basis of the mind, Hobson argues pursuasively against the marination of the brain in drugs. It is when he is extolling "scientific humanism" of this flavour that the compassionate clinician shines through, and the book is at its best.

Susan Greenfield is fellow and tutor in medicine, Lincoln College, Oxford.