She has no language and no memories. She can't see further than 25cm in front of her nose. Her world is sleepy, blurry and tinged with blue. So what is going on in your baby's head?

Most fathers don't keep such a book, of course. Instead they fill hours of video tape, or clog up hard drives and inboxes with digital images. The majority will have parenting books to hand and the advice of in-laws stinging their ears. But in the maelstrom of activity that a new child brings, few will sit down and try to imagine what it feels like to be the baby they dangle lovingly on their knee. If they do, they will come to the same conclusions I did: that a baby's stare is as inscrutable as its reasoning is opaque.

Perhaps the reason for this is that we parents have no memories of early childhood ourselves. We do not recall kicking in the womb. We do not know if we thought or felt during the later stages of our uterine confinement when our senses were almost fully developed (though science tells us we could definitely hear and remember). We have no idea what those first few gulps of air tasted like or even - if our newly-formed synapses were misfiring and we were experiencing some form of synaesthesia - what colour they were. We have no idea what was so interesting about that light switch, why we mispronounced elephant or what it was about bananas that so terrified us.

Where early childhood is concerned, there are many more questions than answers. What was there to dream about when our only needs were food and warmth? How could we think without the facility of language? Did we think at all in any sense we can understand, or was our every move reflexive, the product of biological conditioning or some deeply buried survival imperative? And what did the world even look like when our eyes had an optimal focal length of 25 centimetres and were unable to filter out the short wavelength light rays on the colour spectrum? Blurry and blue is the probable answer to that one.

Most parents don't fill page after page with their observations of childhood, but Charles Fernyhough is one who did. Novelist by vocation, lecturer in developmental psychology at Durham University by trade, the 40-year-old scrupulously documented the first three years in the life of his daughter, Athena, in an attempt to go where few people had gone before: into the world of the very young child. The result is The Baby In The Mirror, a new book subtitled A Child's World From Birth To Three.

Fernyhough's source material is diverse and eclectic. It includes texts such as Thomas Nagel's famous philosophical essay What Is It Like To Be A Bat? as well as works by giants of the developmental psychology field like Lev Vygotsky and Jean Piaget. There is also room for reference to Charles Darwin, Karl Marx, Sigmund Freud, William Shakespeare and the philosopher Immanuel Kant.

But The Baby In The Mirror is also a memoir of sorts, a hymn to a child from a loving father. And that is how it reads. At one point Fernyhough quotes from Sylvia Plath's Nick And The Candlestick, about the poet's son, and from John Burnside's Waking With Russell, written about the narrator's four-day-old baby. Alongside those musings he relates many stories from Athena's own childhood: how he takes his guitar and sings Tom Waits songs to her at eight weeks old (she mimics his actions, windmilling her arm and kicking her feet) and how, two years later, he asks her what she remembers of being a baby. "You know what?" she replies. "When I were a little baby it were very sunny."

I meet Fernyhough at his house near Durham. It is no longer sunny but then Athena is no longer a baby. She is now eight (soon to be nine, her father tells me proudly) and is playing in the living room with her four-year-old brother Isaac. She has just been made student of the week at her school and, for the next four days, will wear a badge saying so. She shows it to me. Meanwhile Fernyhough's wife Lizzie, also a developmental psychologist, busies herself in the kitchen. It's teatime and there is food to prepare.

Over mugs of tea in his study, Fernyhough explains the thinking behind the book. He started writing it when Athena was one, though the note-taking started in earnest as soon as she was born. He filled book after book with his observations, mostly scribbled with one hand while he fed Athena with the other. He describes the notes as "telegraphic", though they were rich enough in detail to enable him to weave a narrative out of his daughter's early life when he finally sat down to write. His other tool was the research done by his colleagues in the field of developmental psychology.

"I have tried to recreate an infant consciousness by saying, What do we know about how infant brains work?' then putting my novelist's hat on, making an imaginative leap and projecting myself into that consciousness," he explains. "Of course I don't know if it's accurate or not but I think it was worth trying to do. Not many people have, particularly in fiction, and I'm fascinated by why children of this age don't appear in fiction. I think it's something to do with us finding this age a bit strange, and challenging, and threatening in some ways."

The world he creates is certainly a strange place to visit and both his research and his observations have caused him to rethink much of what he previously took as gospel. Most importantly, he has come to realise that in the nature versus nurture debate - the question of whether certain character traits and thought processes are hard-wired into us at birth or the result of socialisation - he is swinging far more towards nature.

"My background is thinking about how children's thinking is shaped by their social experiences and I'm still very interested in that point of view. I think there's a lot of important evidence to back it up. But researching the book I had to get into stuff on brain development and behavioural genetics which was newer to me and I found it intriguing and compelling." In other words, he says, "there is something that the child is bringing to the story".

Another of Fernyhough's particular areas of interest is what's known as private speech. This is a monologue which, in adulthood, becomes internalised but which in very young children acts like a sort of running commentary. Hearing toddlers talking to themselves as they go about their toddler business can provide psychologists with a fascinating insight into their developing thought processes as well as their use of language. Interestingly, the children of deaf parents will conduct their private speech in sign language, but what is not clear is whether children with no way of mediating their thoughts - no form of language, in other words - will have their cognitive development impaired as a result. "It's early days with that sort of research," says Fernyhough. "For various reasons it's a difficult thing to study and we just don't know the answers yet. Based on Athena, my hunch is that private speech is important, that it's the way we learn to think. So things that could impact on that - a lack of the right social relationships, an inability to do collaborative thinking with other people like a parent - would have an effect. If you have a problem using language and you didn't have an alternative, you would expect that to have an effect."

An associated subject is crib speech. In one famous study from the 1980s, audio tapes were made of a young girl called Emily when she was alone in her bed at night. "It's really touching to hear what this little girl is saying to herself," says Fernyhough, who has studied the tapes. "She's going over the events of the day, running through what's going to happen tomorrow and running through social exchanges. It's as though she's practising simulating some exchanges she has had or plans to have for herself, off-line as it were."

Crib speech is about storytelling, an important means of processing and organising information which in turn gives on to the rich imaginative life which adults find so charming in children. But it also involves memory, which is a far trickier subject.

It's easy to forget as we go about our busy daily lives, but memory is one of our most important and dearly held attributes. Memory links us to life's entry point - the moment of our birth - and like the golden thread of Greek myth, it unspools behind us as we advance towards the exit, becoming in old age a storehouse of wisdom and knowledge. Collectively, these separate autobiographical threads form a tapestry, the tapestry a history - of a community, a race, a country.

So, with the exception of death itself, there is little we fear more than memory loss, because to lose all memory is to endure a snuffing out of self. Yet the memories we have from the first years of our lives are nebulous to say the least. To return to the analogy of the golden thread, we know it's there somewhere but its first few hundred yards are shrouded in gloom. We cannot see the entrance.

Philosophers, physicians, even theologians have long circled this problem. We can't remember our birth because it was so traumatic and to do so would be dangerous, goes one old saw; we don't recall our early childhood because our souls were inchoate, goes another. It's only recently that psychologists have been able to start unpicking the reasons for our lack of early memories. Now they understand that different forms of information are stored in different ways, and while semantic memory - facts, in other words - does exist in very young children, the subjective memory required for autobiographical detail probably doesn't develop until later.

But if young children have no memories and little language, what do they dream about? Do they dream at all? If they do, can they make the same distinction between their waking and dream lives that an adult would? When Fernyhough's wife asks the two-year-old Athena what dreams are, she says: "Coloured things." At age three, Fernyhough asks her where dreams come from. "Apples," she replies.

Freud, of course, was fascinated by dreams, and argued that they were manifestations of deep desires. The psychologist Jean Piaget studied dreams too and, in a study in which he also asked children where they came from, received answers as varied as: pigeons, the moon, "the smoke under the bedclothes".

One of the most contentious issues in The Baby In The Mirror also involves dreams and memory. Fernyhough merely mentions it in passing, but it involves a paper written by one GA Christos and published in the journal Medical Hypotheses in 1995. In it, Christos theorises that cot death may not have an enviromental cause at all. Instead, as the new-born baby enters REM sleep, he postulates that it starts to dream and, as it has no memories of life outside the womb, it dreams of life inside it - and, of course, babies don't breathe in the womb.

"Taking the idea seriously is not the same as saying it's true," says Fernyhough. "It was an interesting theory that I don't think has any basis." Besides, as he points out in the book, even if it were true there are enough safety mechanisms built into the brain's subcortex to ensure cot death is rare. Still, the thought that dreaming could be fatal is not pleasant.

Suddenly the door to Fernyhough's study opens and Isaac enters, a little shy but unwilling to leave despite his father's remonstrations. He has a drawing of a butterfly he wants to show him. He climbs on to the sofa, then on to Fernyhough himself. Then Athena appears. "Do you want to come and do a puzzle with me, Isaac?" she says, trying to lead her brother away. She is tall, bright, sparky and pretty.

Athena has already told me that, although she hasn't read the book yet, she will try to soon. She seems unfazed by being its subject, but so far she has only told one friend at school about it. When she and Isaac leave, I ask Fernyhough if he had any misgivings about using his own child's early years as source material.

"Yes, I did," he says. "The ethics of it worried me from the start I was worried about whether it was placing her too much in the public eye and I was worried about what she would think of it in the future. I still have no idea what she will make of it when she is a teenager, but I hope that when she's grown up she will enjoy it and appreciate it and understand why it was done."

One thing he was fastidious about was making sure Athena was involved with the project all along and that, as much as a toddler can, she understood why the notebook and the camcorder were ever-present. Today, though, Fernyhough says he is happy with the ethics of the project. Fundamentally, he thinks he has done a good job and that the motives behind the book are good and clear. And, he adds, when Athena reads it as a grown-up - and, perhaps, rereads it when she has children of her own - she will see it for what it really is: truly a labour of love.