Scrolled out to its full extent, the photo stretches from one end of the kitchen table to the other. An entire school - 600 boys - unfurled from 1964. “That’s me,” Tom says. He’s been tapping his finger along a row of monochrome heads. “Third row down, 39 from the left.” He does not recognise the lank-haired boy in the picture, or any of his classmates. They all look the same to him. As do the teachers in the front row. Tom, one of my patients, is prosopagnosic - face blind - and has, on occasion, failed even to recognise his wife and his children. Does he recognise himself in the mirror, I ask, immediately realising the absurdity of my question. “Well, who else could it be?” he says.

But it occurs to me that we never really see ourselves in the mirror. There is a famous painting by Rene Magritte. It shows, from behind, a dark-suited man looking into a large mirror. The reflection in the glass shows the same rear view. The man is looking directly at the back of his head. The image is unsettling because it invites us to consider that when we look into a mirror we are not looking at ourselves, but at a mere representation of ourselves. What you see is the image of a physical object: a head. And a head is not a self.

No one thinks that heads are precisely equivalent to selves, you might say, but surely I am in there somewhere. Take an imaginary journey into the space behind the face, through the eyes and into the brain. Do you see any sign of yourself? What you actually find is material substance and total darkness. It still gives me a peculiar thrill to contemplate the interior of my head in this way. It’s as if I’m imagining myself out of existence, reducing myself to a lump of meat rather than a person.

The weirdness of “meat-based mentality” is captured brilliantly by Terry Bisson in his science fiction story They’re Made Out of Meat. An alien spacecraft visits Earth on a reconnaissance mission. One of the explorers reports back to his disbelieving commander:

“They’re made out of meat.”

“Meat?”

“Meat. They’re made out of meat.”

“Meat?”

“There’s no doubt about it. We picked up several from different parts of the planet, took them aboard our recon vessels, and probed them all the way through. They’re completely meat.”

“That’s impossible... You’re asking me to believe in sentient meat.”

Up close, but not personal

Actually, in its natural state, encased within the skull, brain matter is more gelatinous than meaty. Egg yolk provides a better comparison. But it is material substance just the same - all the way through. Look closely and see for yourself. At microscopic level, you can see that the brain is a dense matrix of intercommunicating cells, or neurons - more than 100 billion of them. The potential number of connections between them exceeds the number of elementary particles in the known universe. Now shrink down to the size of one brain cell and picture yourself among these neurons, perched in the branch-like connections like a monkey in a tree. Can you see anything around you resembling a soul or a self, or even a thought? Where now do you imagine the conscious self to be? What can you possibly be trying to imagine - some ghostly essence wafting through the jungle?

Gottfried Leibniz, the 18th-century philosopher and mathematician, performed a similar thought experiment. He imagined “a machine whose construction would enable it to think, to sense, and to have perception” and, further, that the machine is “enlarged while retaining the same proportions, so that one could enter into it, just like into a windmill”.

What does he find in the interior of the mind-making machine? “I only parts pushing one another, and never anything by which to explain a perception.”

From this perspective of brute biology, it’s plain to see that the brain is nothing more than a vast congregation of mindless neurons. But we believe there is more to us than that. It’s hard to identify oneself with the flux of neuronal activity, no matter how complex the patterns. Most people are reluctant to embrace what Francis Crick (co-discoverer of the structure of DNA and, these days, consciousness researcher) has called the Astonishing Hypothesis: “that ‘you’, your joys and your sorrows, your memories and your ambitions, your sense of personal identity and free will, are in fact no more than the behaviour of a vast assembly of nerve cells and their associated molecules”.

Two theories of “self”

What then, is a “self”? Broadly, there are two views on the matter: ego theory and bundle theory. Ego theory represents the intuitive, common-sense view. There is an “I”, an experiencer of experiences that constitutes the essential core of every person. The philosopher most closely aligned with this view, Rene Descartes (1596-1650), believed that our capacity for self-awareness is due to the possession of an immaterial soul. It is this that gives us unity as individuals and continuity over time.

Bundle theory has origins in Buddhist teaching, but owes its modern formulation to philosopher and historian David Hume (1711-1776). It rejects the idea that actions and experiences are owned by some inner essence, ego or “I”. There are just sequences of actions and experiences. Nothing more.

Each life is a long series - or bundle - of interconnected mental states and events. “For my part,” said Hume, “whenever I enter most intimately into what I call myself, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I can never catch myself at any time without a perception, and never can observe anything but the perception.”

According to this view, the self is no more than a constellation of fleeting impressions. For Hume, the extension of the self beyond such impressions was a fiction. The philosopher Daniel Dennett offers a contemporary view of bundle theory. He emphasises the power of language in binding our experiences together over extended periods - and thus creating a continuous sense of self. Seen this way the self is fundamentally a kind of story, but it is not so much a question of us weaving the stories as the stories weaving us.

Are you in the ego or the bundle club?

If you are uncertain as to whether you are in the ego club or the bundle club, try the following thought experiment, devised by the Oxford philosopher Derek Parfit. In the future you find yourself taking business trips to Mars. The journey, by conventional spacecraft, takes months. Then someone invents a teleportation device, a “beam-me-up” machine just like in Star Trek. You enter a booth and press a button, and before you know it you are stepping out of a similar booth on Mars. Travelling at the speed of light, the journey takes a little more than four minutes, but to the traveller it seems instantaneous.

It works like this. A scanner plots the co-ordinates of every atom in your body, and digitally encodes the information for radio transmission to Mars.

In the process your body is destroyed, but reconstructed using locally available materials as soon as the radio signals are decoded. The traveller arrives on Mars in precisely the same condition that heshe departed Earth, with an identical body and brain, and identical patterns of mental activity, including memory systems replete to the last atom and iota of information.

Are you comfortable with this scenario? If so, you are a bundle theorist.

You reject the notion of an inner essence or soul. The observing “I” - the “self” - is no more than a pattern of actions and experiences bundled together by the operations of the central nervous system. Such patterns can be disrupted and reconstituted without destroying the “self”, because there is no self to destroy - nothing separate from the neuronal activity. The patterns are all.

If, on the other hand, you decline the option of teleportation, believing that some essential “you” is lost in the process of destroying the scanned body, then you are an ego theorist. You imagine that the person striding out on to the Martian plain is not you at all, but a mere replica. It will have all your memories, skills, hopes, fears, and expectations. Indeed, it will swear it is the very person who stepped into the booth on Earth. But, you say, it could not be you, because your body and brain have been destroyed.

Note the subversion of conventional thinking. Those who believe in an essence (or soul) suddenly become materialists, dreading the loss of the “original” body. But those who don’t hold such beliefs are prepared to countenance a life after bodily death.

The lady vanishes

As a clinical neuropsychologist, helping patients and those around them come to terms with the psychological consequences of brain disorder, I see problems of self-awareness and personal identity not just as conundrums of science and philosophy, but as matters of real practical concern. At the same time, the experiences of neurological patients may illuminate some of the deepest problems of philosophy. “I know that I exist,” said Rene Descartes, “the question is, what is this ‘I’ that I know?”

One of my patients, Stella, who has temporal-lobe epilepsy, often experiences a similar quandary. Her seizures start with a feeling of detachment. There are changes in the quality of her perceptions. Colours seem more vibrant than usual, objects and people stand out with unusual clarity. At the same time, everything around her begins to feel utterly unfamiliar. She tries hard to resist the next phase, but usually fails.

“Then I sort of evaporate,” she says. “I have no idea who I am.”

Her identity has vanished. She does not know her name and has no recollection of her personal history. Nor does she have any appreciation of where she is or what she is supposed to be doing. These episodes are brief and, within a few minutes, full self-awareness is restored.

When a seizure strikes, Stella loses her identity, yet still retains some sense of self. Her “evaporations” seem to lay bare what is sometimes called the “minimal” or “core self” - the self of the present moment. In the words of neurologist Antonio Damasio, this core self is “a transient entity, re-created for each and every object with which the brain interacts”.

This is the most primitive feature of self-awareness. But in certain rare cases even the core self seems to dissolve. Another of my patients believed that she had ceased to exist. “Am I dead?” she asked. She was unconvinced when I told her that she was, in fact, alive - that we were drinking tea and having a conversation. “What about you?” she said. “Are you real?”

This condition is known as Cotard’s syndrome and may be due to a neurological decoupling of feelings and thoughts. Thinking that one exists is not enough, the notion must also be felt: “I feel I think, therefore I am.”

“Get this head off me”

Ordinarily, when we think of ourselves we have something other than the primitive core self in mind, something closer to the notion of “personal identity”. We think of a more elaborate being, with stores of knowledge and personal memory, and with dispositions to act in certain ways. This is the so-called “extended” or “autobiographical self”. It is, in some ways, disconcertingly fragile. There are many ways of wiping the neural records of a life: the slow progression of Alzheimer’s disease, the rapid devastation of a viral infection blasting through the brain’s memory circuits, or the deep-brain wreckage of severe alcoholism.

Yet, in other respects, the autobiographical self is remarkably robust.

Another of my patients, Judy, came round from a heavy sleep to find that a stroke had reset her brain’s calendar 23 years back to 1976. She had no recollection of the intervening years. The strange man in the room turned out to be her second husband. When Judy looked in the mirror she saw her face was wrinkled and her hair was short and grey. But, despite the severe autobiographical discontinuity, integrity of personality was preserved. She was “the same Judy”, and in time she adjusted to her new circumstances and came to accept the new version of her story.

Other forms of neurological disorder have an opposite effect, leaving memory more or less intact, but recalibrating the personality. Jeff was hit head-on by an overtaking car. I remember him on the ward in the first few weeks, furiously wrenching his neck. “Get this head off me,” he kept saying. “Get it off, it’s the wrong one.” The image haunts me. The original head housed the steady disposition of a loving husband and father, the knowledge and skills of an educated man. His damaged brain is now a place where thoughts and urges roam untethered, provoking dark turns of mood and, sometimes, spiteful outpourings of abuse. “I tell myself it’s not really Jeff,” his wife says. But she stands by him out of the dutiful conviction that, at some level, it is really Jeff.

A stern test

I have long since rejected the myth of the immaterial soul. Few scientists these days would agree with Rene Descartes that people are made of two kinds of stuff: mental and physical. (Are we to imagine a mutilation of Jeff’s soul as well as his brain? And, if not, what does the pristine soul make of his disturbed behaviour?) But I find it harder to shake off “the myth of the self” - that sense of there being an inner “I” monitoring the screens of perception, orchestrating thoughts and actions. As a neuroscientist, I am intellectually inclined towards the bundle theory.

Like most other people, I live “the myth of the self” - JI buy the illusion of the inner “I”.

But I pass the simple teleportation test. I’d be prepared to give it a go.

Unfortunately, there’s another, much sterner test for those who wish to enter the inner circle of the bundle club.

Imagine this. You are now quite used to being teleported to Mars, having made the trip a dozen times. But then there is a problem. About to make your thirteenth voyage, you step into the scanner and press the button, but when the door slides open you realise you are still on Earth. There’s been a serious malfunction. The scanner actually read your atomic co-ordinates and transmitted the information to Mars, but failed to vaporise your body in the usual way. Your replica was automatically constructed and is happily going about its (your) business. Worse still, the faulty scanner has left you with a fatal heart condition. You will be dead within days. How does that make you feel? Not so good, perhaps.

“But what’s the difference?” says your counsellor. “Suppose all had gone according to plan. You would have stepped into the booth, the scanners would have read your atomic profile, your body would have been vaporised, and your replica would have appeared on Mars. And that is what has happened - that’s what always happens - except this time the demise of your Earth-bound body has been a little delayed.”

“Look,” you tell her, “I can’t be the person on Mars, because I’m down here on Earth. There can’t be two of me, can there?” She puts you right. “Well,” she says, “you shouldn’t expect determinate answers to questions about personal identity: the ‘same’ person or not? One person or two? Persons are just not that kind of thing. They are no kind of ‘thing’ at all, in fact.

And why do you think personal identity matters in the first place?”

You should think of travelling by teleportation as no more threatening or problematic than travelling, via the oblivion of sleep, from one day to the next. What matters in both cases, in terms of what is preserved, is precisely the same: psychological continuity. We are the same from one day to the next only in so far as the bundle of mental states that our brain takes with it to sleep at night resembles the bundle that it wakes with in the morning.

I’m not at all sure I would willingly enter into this second scenario, even though there is no logical difference between this state of affairs and that described in the first scenario, at least in terms of the personal trajectory of the individual arriving on Mars. Perhaps I am not ready for the inner circle of the bundle club. Perhaps, as Derek Parfit seems to claim, to feel comfortable with the idea of psychological continuity in the face of material oblivion is to achieve a kind of enlightenment.

The evidence of neuroscience is at odds with our deepest intuitions.

Neuroscience reveals that the mental processes underlying our sense of self - feelings, thoughts, memories - are scattered through different zones of the brain. There is no special point of convergence, no inner sanctum of the ego. And neurological case studies reveal “the self” to be multifaceted and fragile.

We are all just a stumble or a burst blood vessel away from being someone else.

For general information about neuropsychology visitwww.neuropsychologycentral.comwww.neuropsychology.co.ukContemporary Philosophy of Mind: An Annotated Bibliography is compiled by David Chalmers of the University of Arizona:www.u.arizona.educhalmersbiblio.htmlThe Whole Brain Atlas can be found by visitingwww.med.harvard.eduAANLIBhome.html

Paul Broks is senior clinical lecturer in neuropsychology at Plymouth University. His book, Into the Silent Land: Travels in Neuropsychology (Atlantic, pound;14.99) was shortlisted for this year’s Guardian First Book Award