The human brain, Veronica O’Keane tells us, contains 68bn neurons. The fact is easily stated, but its consequences are … well, mind-boggling. Neurons, or nerve cells, register and process information from the physical world, and control the functioning of the body by electrical messaging. They connect to each other by way of dendrites, and the cell assemblies fire together as a unit. Since each neuron can have up to 15,000 dendrites, the connective possibilities are, as O’Keane remarks, “virtually infinite”. The firing and wiring together of cell assemblies create memories. “The key process in forming even a short-term memory is that the cells have to fire together for long enough to become wired together. The firing together forms a transient memory and the wiring a more permanent memory.” Simple. No, of course it’s not. The functioning of the brain is the great wonder of the world, and until recently we could do little but regard it with helpless awe. Neuroscience has made rapid progress, however, not least because of advances in other disciplines, such as computer technology, and even particle physics. O’Keane is a professor of psychiatry at Trinity College, Dublin, and a practising consultant psychiatrist. As a working scientist, rather than a theoretician, her interest lies in what the brain does when it operates as mind – though she would reject the distinction: “Brain and mind,” she declares, “are one and the same” – and what happens, and why it happens, when the mind loses its way in the haunted forest of everyday experience. She punctuates her narrative with brief but unforgettable case histories of some of the patients she has treated in her work as a clinician. These passages are vivid and immediate, and all the more affecting for the measured and unemphatic manner in which they are set down. If O’Keane is as fine a doctor as she is a prose stylist, her patients are fortunate indeed. The first “story” she tells is that of Edith, whom she encountered when she worked at the Bethlem Royal hospital in London in the early 2000s. Edith had no history of mental illness until she gave birth, but thereafter, the onset of her illness was rapid and devastating. “Like most women in the throes of postpartum psychosis she appeared to be in a state of altered consciousness, as if removed from the world.” A symptom of her psychosis was her conviction, sparked in her by phantom voices in her head, that the baby she had given birth to had been stolen and substituted with an identical “changeling” – a common delusion that the author returns to at the very end of her book, neatly closing the structural chain. Hearing a human voice is a subjective experience, whether the voice originates in the outside world or in the brain In time Edith responded to antipsychotic medication, and was able to return to normal life with her baby and family. In subsequent check-ups she confessed that she was ashamed of having rejected the baby while she believed the infant had been stolen. O’Keane explained to her that the psychosis was the result of hormonal changes occurring during the birth, which gave rise to “subjective experiences that seemed to come from the outside”. Here we encounter the “first principle that we need to establish”, which is “that what are called symptoms are real sensory experiences. Hearing a sound, a human voice, is a subjective experience, whether the voice originates in the outside world or is generated in the brain by pathological neural firing.” The voices Edith heard were imaginary, but that did not mean her hearing them was not an actual experience. It may seem an obvious point, but it is frequently discounted by those caring for and treating people with mental health problems. Sometimes it is the case that the more we know, the more we forget. And by now we do know a great deal about how the brain works, though our knowledge may not be as comprehensive as O’Keane suggests. She is impatient with the lingering notion that “some human experiences are ‘psychological’ and some are ‘organic’.” For her, the thing is decided: “In the real organic life of the human, brain function and matter are indistinguishable, because every experience in the brain, whether a normal or an abnormal one, is based in matter and how that matter functions.” But is the case as sharply cut and dried? To state that “brain function and matter are indistinguishable” is to glide over the ‘hard problem’ of consciousness. This was identified in the mid-1990s by the Australian cognitive scientist David Chalmers, and has provoked a great flurry among scientific and philosophical dovecotes. To put it simply – perhaps too simply – the question Chalmers posed is, how can consciousness arise out of non-sentient matter, such as neurons and dendrites? In other words, how can a lump of squidgy grey matter think? Or to approach more closely to O’Keane’s central subject, how do our physical brains store phenomena as shimmering and evanescent as memories? Numerous solutions have been advanced, but the hard problem is as hard as ever. How would it not be? In other formulations it was already old when the pre-Socratics tackled it. But this is to quibble. O’Keane has written a fascinating, instructive, wise and compassionate book, the significance and value of which it would be hard to exaggerate. The authorial voice is measured and humane, the range of reference is wide, especially in literature – from Proust to John Berger, from William James to Samuel Beckett – and the clinical and philosophical speculations are as stimulating as they are provocative. In these pages there is much for the reader, or at least for this reader, to learn, but there is also a lot that is simply delightful. For instance, did you know there is a hormone that influences the onset of puberty in girls called kisspeptin, or KiSS? For more of the like, inquire within.