I think, therefore I am.
René Descartes

I am what I am. That’s all that I am.
Popeye the Sailor Man

I have a friend who suffers from schizophrenia. Voices whisper and bark around her head. Malign forces emerge from the internet to control her thoughts. She tries to resist but, looking in the mirror, she often sees nothing more than a vacant machine. It’s a psychotic delusion, but also a glimpse of the truth. Look at your own reflection. What do you see other than machinery? Your face is an animated device attached to the outer surface of a bony box. Concealed bands of fibre tug the surface tissues this way and that. You may imagine that the thinking, feeling “you” is somewhere inside the bony box. But if you looked inside, you’d just find more machinery.

The gelatinous substance of the brain is a dense matrix of billions of robotic cells. Inside the cells are other intricate machines. There’s no one in there. So how does the conscious “you” clamber from the darkness of the box out into the bright arena of subjective experience? There are two puzzles to solve. No one has yet fathomed how the material substance of the brain can conceivably give rise to conscious awareness. This is known as the “Hard Problem”. Neuroscientists remain upbeat about a solution, but some philosophers have given up the ghost. Transmutation of meat to mind is alchemy beyond the scope of human intellect. The other puzzle is more amenable to science: how does the brain, with its diverse and distributed functions, arrive at a unified sense of self? As a neuropsychologist, I have a professional interest in the reverse process: when the machineries of the brain are damaged by disease and injury so the self disintegrates. It can be a fading to oblivion through Alzheimer’s disease or the sudden calamity of a stroke or head injury. Either way, the self is at stake as well as the brain.

Modern neuro-imaging methods, combined with traditional “lesion studies” (correlating brain damage and behaviour), are producing ever more refined models of the working brain. We know a good deal about how different neural systems process sensory information; we also have solid knowledge about the location of mechanisms that drive language, memory and emotion. But when it comes to the neural circuitry of the self we arrive at a conflict between our natural intuitions of selfhood (that mysterious essence occupying the space behind the face) and the facts of brain science (the vacant machinery).

There’s a paradox here. We recoil from images of ourselves as soulless machines, yet we often equate “the self” with the machinery of “the body”. We all own a body and claim sovereignty over it, but when we speak of the self we usually have something else in mind — a bundle of thoughts, feelings and emotions, overseen by an ethereal, observing “I”. Patients who have spent days or weeks in coma are often perplexed by the question, “Where was I?” not, “Where was my body?” but, “Where was I, the person?” — to which the honest, but disconcerting, answer is “Nowhere”.

The neurologist Antonio Damasio distinguishes the “core self” from the “autobiographical self”. The core self is rooted in the present moment, “a transient entity, re-created for each and every object with which the brain interacts”. It is underpinned by neural systems that perform moment-by-moment mappings of bodily activity. The autobiographical self is a mental time traveller, reflecting on the past and reaching to the future. It’s closer to the notion of “personal identity” and relies on a different set of brain systems, especially those involved in memory and language. Professor Damasio’s work sheds light on the neural infrastructure of the “sense of self”, but it also sharpens some of the paradoxes of selfhood. The brain is revealed to be precisely what we are — and precisely what we are not. The grey, materialistic picture I have painted of the brain as vacant machine merely provides the backdrop to the real stuff of the self, which is storytelling and imagination. We are not that soggy mass of robotic cells, although we depend on them. We are, rather, the tales they tell.

Professor Damasio might have referred to the “novelistic self” rather than the “autobiographical self” because the self is fundamentally an act of imagination. The neuroscientist Michael Gazzaniga locates the storytelling machinery in the brain’s left hemisphere: the function of “the Interpreter”, as he calls it, is to identify patterns of connection between different brain modules and correlate them with events in the external world. The activity, internal and external, is wound into a single narrative thread of subjective experience. This gives us our sense of unity and continuity.

Dr Gazzaniga’s “Interpreter” operates largely outside of conscious awareness, as do most of our mental activities, but we have an “inner voice” — a subterranean babbling brook of words — that helps to bind the narrative at a conscious level. Storytelling is built into the fabric of the brain and if it’s not our own story we are attending to we readily latch on to others. You are doing it now, reading these words. My reason for writing them is precisely to take charge of the little voice in your head. You happily relinquish control. It feels satisfying to submit to the guidance of another voice. Think of a green giraffe. See, it’s irresistible.

The psychologist Julian Jaynes claimed that only since the second millennium BC have human beings felt themselves to be the authors of their own thoughts and actions. Before that, behaviour was directed by hallucinated voices perceived to be of supernatural origin (although actually arising from the right hemisphere of the brain). Achilles and Agamemnon were puppets of the gods. To lose oneself in a book, to be beguiled and steered by the authority of another voice, is almost to enter a comparable state of hallucination. But if inner speech helps to create a sense of personal identity, what happens when the voice falls still? Shortly after Scott Moss, a 43-year-old clinical psychologist, was appointed to a post at the University of Illinois, he suffered a stroke that virtually abolished his capacity for speech. He lost the ability to converse with others and to engage in self-talk. He later wrote of the process of recovery, describing the condition of total wordlessness as being like confinement to a continuous present: “I did not have the ability to think about the future, to worry, to anticipate, or to perceive it. . . I simply existed. . .”

The biological basis of selfhood raises matters of social and moral concern beyond the personal trials of neurological disorder and mental illness: the issue runs deep through the debates on abortion, cloning and stem-cell research. When should moral status be conferred upon a human embryo in the journey from the brainless clump of cells that is a fertilised egg to the eager bundle of love that is a newborn baby? Sadly, neuroscience cannot offer definitive guidance. By four weeks post-conception a part of the embryo, the neural tube, has begun to develop three bulges corresponding to major structures in the mature brain. Two weeks on, the cells are showing primitive electrical activity — but less coherent than that in the nervous system of a shrimp. By 23 weeks the foetus possesses a viable nervous system. Everything is in place for the emergence of a thinking, conscious human being.

But there is no capacity for self-awareness. The hardware is functional, but the requisite software has yet to be installed. Some psychologists and philosophers — Daniel Dennett, for example — argue that conscious awareness reaches full bloom only with the acquisition of language. On this account animals and pre-linguistic children lack self-awareness. Dennett insists that neither is thereby morally demoted, but does this mean it is more acceptable to eat small children or less acceptable to eat animals? The obliteration of the self is more straightforward. Once the brain irreversibly ceases to function, so does the person. This is of no concern to those who believe in an afterlife. The conscious personality just relocates. That most people hold to this belief is not due simply to religious indoctrination. We are mindreaders as well as storytellers, adept at extracting unobservable thoughts from the behaviour of observable bodies, and automatically inclined to picture an “inner essence” directing the action from behind the eyes. This enables us to negotiate the complexities of interacting with other people and make sense of what is going on in our own heads. Autistic people, with impoverished mind-reading skills are at a severe disadvantage.

So, too, are people such as my schizophrenic friend, who generate rich, but unreliable, interpretations of their own and others’ mental states. The propensity to divide self from soma is hard-wired into the brain. Take the next step of imagining that mind and body are separable and it’s easy to see the possibility of a mental life after physical death. But that’s another story. . .

Paul Broks is the author of Into the Silent Land, Atlantic Books, £8.99