Susan Greenfield, professor of pharmacology at Oxford University and Director of the Royal Institution, is worried. She fears that galloping technological advances, and the social changes they are bringing about, will not only transform our sense of who and what we are, but might alter our identity to the point where we may no longer have the capacity to be fully developed persons.

Because she is a neuroscientist, Greenfield poses her questions, and frames her search for answers, in neuroscientific terms. “Our identity,” she says “is our brains”; more specifically, it lies in “the personalised connectivity of an otherwise generic brain”. Brains are plastic and this lies at the root of her concerns. They respond to experience by changing the way their neurones are wired together - the number of connections and their strength. This is how we mature and acquire those skills that enable us to function in the world. But its plasticity also makes the brain susceptible to unwelcome and unforeseeable influences. Twenty-first-century technologies may bend our brains, and hence erode our identities, in ways previous generations could not have envisaged.

In her exploration of the possibilities opened up by new technology, Greenfield contemplates psychoactive drugs, stem-cell treatments, direct brain stimulation, face transplants and the use of neurons grown on integrated circuits to repair damaged brains or enhance intact ones. Less futuristically, she also airs highly topical concerns about children spending hours glued to a computer screen, playing violent electronic games, cultivating an identity on the net, making net-friends or simply cheating by downloading project material. The evidence as to whether young screen addicts will turn into e-hermits, whether or not their attention span will be reduced, whether their hours in a secondary reality that exacts no penalty will make them greater or lesser risk-takers, is equivocal.

The absence of conclusive research does not, however, prevent Greenfield from speculating that a diet of addictive fast-paced games might so radically reprogramme our grandchildren's brains that they will not see themselves as individuals at all. Blurring the boundaries between the actual, the probable and the merely conceivable, she bombards us with all sorts of alarming suggestions; claiming, for example, that the monitoring and profiling to which we are today subjected is putting our mental inner sanctum under threat and taking us back to the world of our infancy where Mummy knows everything about us. She even wonders whether “the old dichotomies that have held for every human society...the real versus the unreal...the self versus the outside world” are melting.

iD offers more questions than it provides answers. This is, in part, a reflection of Greenfield's honesty. But it also reflects her failure to formulate her worries clearly: the impression is of someone circling round an unidentified object in a bag. She does not make clear what she means by personal identity; nor, more importantly, what she feels is at stake when she worries about its future. Her brief allusions to “mind” (which she equates with self-consciousness) as the first-person perspective of identity and “personality” as the third-person perspective are hardly illuminating. She scarcely touches on real and interesting questions about subjective identity and objective identification and the relationship between them. Nor does she adequately address the nature of the continuity of identity over time. She glosses over the contributions of psychological continuity (through, for example, memory) and physical continuity through the body. The absence of the names of the leading philosophers of personal identity in the index is telling, as is her failure to engage with the huge literature on the social construction of the self.

More damagingly, Greenfield fails to justify the central place she gives to brain science. Much of her engaging and highly accessible account of contemporary neuroscience seems to have been included merely because she is a neuroscientist. Her claim that identity is the product of the brain personalised through the connections formed by individual experience is a bit of a cheat. “Customised” would be fine but “personalised” begs the question of how impersonal processes could add up to a person.

I am not persuaded that neuroscience can give one an account of a personal identity worth having: the processes she describes are no more personalising than the events that happen in any nervous system, including those that belong to organisms to which nobody is inclined to ascribe personhood, such as dogs and monkeys. While she acknowledges the “hard question” of how neural activity gives rise to any kind of consciousness, she overlooks the equally hard question of the relationship between what goes on in our individual brains and what happens in that vast space of possibility that is the human world, the community of minds where our identities are forged. The socially constructed self requires a brain in working order but that is the beginning, not the end, of the story. Brain science can consequently tell us very little about being a person. Her brain fixation prevents Greenfield from putting the changes that worry her into proportion or addressing their implications at the level they demand. She admits that there are no patterns of activation corresponding to beliefs, but still asks whether “buying into a strongly held belief system [might] change your brain, and hence your mind, and hence your identity?” I don't think we need to invoke putative changes in the cerebral cortex to decide whether your identity might be changed by conversion to Scientology.

Greenfield's neural account of personal identity is, despite her claims, profoundly reductive, if not incoherent. As such, it removes the very premise upon which her book is based: that technological advance poses a unique threat to personal identity. For if our identity were truly to be reduced to patterns of brain activity, it should not matter whether what shaped the brain was wind and rain, brown ale and books, or stem-cell implants and computer games: the helplessness and emptiness would be the same.

iD: The Quest for Identity in the 21st Century by Susan Greenfield
Books First price £15.29 Buy the book

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist