Raymond Tallis is frequently cited as one of our foremost thinkers. This is fortunate because he would never have made a career out of his looks. At 61, he has a sunken-eyed face, large ears that look as though they were clamped on as an afterthought, and an ungenerous mouth. The pen-portrait, I should add quickly, is his, not mine. He has used it as the opening salvo of his latest book - a paean to our heads that is mind-blowingly cerebral but not nearly as boring as you might think.

The last time Tallis featured on these pages was four years ago, when, after a career dedicated to the National Health Service for more than three decades, he published a prolonged analysis of its problems and a caustic critique of those he believes are to blame for them. In Hippocratic Oaths he upbraided everyone from Dr Andrew Wakefield (of MMR notoriety) to unquestioning consumers of alternative therapies and the “unthinking voices” whose shallow understanding of medicine, he believed, threatened irreversibly to corrupt his beloved profession. Writing it was, he says, “a combination of therapy and a farewell to arms”.

Having elegantly vented his discontents, in 2006 he resigned from his post as Professor of Geriatric Medicine at the University of Manchester and retreated to where we are now - the upstairs study of his Stockport home - to set about fulfilling a long-held dream of becoming a full-time writer.

At first, he concedes, there was definitely too loud a silence. He had been used to queues of people outside his office and brisk ward rounds in which students and patients hung on his every word. “All that had given me a sense of pleasurable selfimportance,” he says. “But it is also harassing, constantly fielding the interruption to the interruption while trying to be courteous, and to give time to Mrs X who has just burst into tears because you've had to tell her something terrible, while the nurse is reminding you that you have a full clinic waiting.” The harassment he doesn't miss at all; the solitude he soon learnt to relish.

Tallis is a compulsive writer and his study shelves groan with his medical publications - major textbooks as well as scientific papers, which sit alongside his acclaimed works in existentialist philosophy and cultural criticism. Just looking at their spines gives me a doomy feeling. There is no hope of my pretending to have scratched the surface of their contents, let alone to have understood the theories that they propound. Prospect magazine lists him as one of Britain's Top 100 public intellectuals, while another newspaper last year named him one of 50 “Brains of Britain”. Suffice to say that he is the Lennox Lewis of the intellectual world - a formidable heavyweight. Which is why it is such a surprise to find, in his latest offering, an entertaining lightness of touch.

The Kingdom of Infinite Space is a book that could have been conceived only by a mind as original as that of Tallis. His idea has been to take us on a journey around our heads, focusing not on the brain, which he claims is “absurdly overrated”, but instead on the head's myriad functions - laughing, crying, sneezing, listening, talking, breathing. He analyses, often in nauseating detail, the mechanics of mucus, vomiting and sweating. He wittily explains the geology of a blush and the complexities of kissing. Underpinning everything about which he writes - from the infectiousness of yawning to the hinterlands of a nod and a wink - are two things: a high-minded philosophical inquiry into human consciousness, and a boyish wonderment at the utterly brilliant entity that is our heads.

He tells me that The Kingdom of Infinite Space was his first writing project after his retirement from the NHS, and certainly his sense of liberation and newly discovered playfulness shows. He wanted, he says, to provide “a corrective to the headless brains that fill the newspapers” (ouch!) and to dispel the myth that the essence of what we are is to be found in the brain. As he began his research, he admits: “There were times when I found myself chuckling out loud”. He was tickled, for example, to discover that Inuit populations were once tracked across the snow by the consistency of their earwax, and that the tears you cry as you read Tennyson are different (richer in manganese and protein) than those provoked by a poke in the eye. These are anorak facts, but Tallis manages to present them with the sort of exuberance that, I can't help thinking, is needed in our classrooms if we are to redress the alarming dearth of science graduates emerging from our universities.

A recent international study found that British 15-year-olds' science knowledge had slipped from fourth to 14th place in world rankings within the past six years. Tallis knows about this. Indeed, he tells me, it is something about which he feels passionate.

As the mother of a rising 15-year-old who finds science a slog, I wonder what he recommends. He is clear that the answer is not to entice pupils by focusing only on the subjects in which they are already interested, such as global warming. “Children should struggle. They need to be taken to a place that is hard to get to,” he says. I don't want to contradict him - he is, after all, the big brain in the room - but what has made his latest book work for me, a science airhead, is the fun factor. “You're right. I'm being Calvinistic. You have reminded me of something I'd forgotten,” he says, suddenly animated. “As I child, I spent hours looking down a microscope.”

Born in Liverpool, the middle child of five, Tallis is the son of a builder. His father, a self-made man who put all his children through private education, was inspiring but also tough, controlling and a worrier who believed that “the world was always ready to drag you down”. This anxiety rubbed off on the young Raymond, who worked hard and loved science because it enabled him to take delight in Nature's “hidden mysteries”, but also became an angst-ridden adolescent, preoccupied with suffering, death and a fear of the meaninglessness of life. In search of an answer, he turned to philosophers - Plato, Leibniz, Spinoza and his favourite, Heidegger. As he absorbed their thoughts, he found an escape for his own in writing. He composed his first novel at the age of 12: “It was a mass of clichés, but I remember the joy of suddenly realising that you could write your own books.”

Tallis might easily have eschewed medicine altogether had he not felt the need to “do something useful”. He went up to Oxford at 17 but failed to take a First after spending the three days before his finals reading Dostoevsky's The Devils “as a minor act of rebellion”. His plan had been to use medicine as a stepping stone to a career in biochemistry. “What I hadn't catered for was the fact that I was no good at it.” Biochemistry is a hands-on science. He is so impractical that, even after 37 years of marriage and the raising of two grown-up sons, he confesses that he would struggle to boil an egg. So he stuck with medicine and found himself gravitating towards gerontology.

Geriatric care has long been unfashionable but Tallis became its champion, reorganising neurological services in Greater Manchester and setting up a specialist epilepsy service for older people. “I saw that there was a disgraceful gap. People weren't being cared for and ageism was the problem.” He would delight in telling his medical students how boring young people were: “Older people are endlessly fascinating because they have 70 years of experience coiled up inside them.”

Despite the disappointments that he chronicled in Hippocratic Oaths, he was consoled by the fact that he left geriatric medicine better than he found it.

Tallis's wife Terry is a social worker in adoption and fostering, and he has often reflected on how much more difficult the decisions that she makes are compared with those that he made in medicine: “At least I could always measure the serum potassium to find out what was going on.”

He speaks movingly about his great debt to Terry for allowing him, throughout their marriage, the luxury of time to write. For more than 30 years he rose at 5am to work for two hours on his books before going to the day job. On family holidays he would take the morning to work on his books, then look after the boys (now 28 and 23) in the afternoon.

Today, Tallis enjoys a distinguished reputation as a philosopher, poet, playwright and novelist, but it has been hard won. He estimates that he received 135 rejection slips before he was first published in 1987. What was he doing wrong? “The vain reason is that my books were ahead of their time. The real reason is that they probably weren't very good,” he says wryly. “One of the hardest lessons of writing is to know your reader. I thought it was good enough that he or she should have this amazing work. I was thinking ‘this should be difficult' and not appreciating that there was nothing wrong with being engaging and pleasurable.”

Tallis has five more books in progress, including De Luce, the magnum opus on which he has been working since the 1970s. He has written two and a half million words on the possibilities of human consciousness, dumped a lot of them and is now aiming for between 1,000 and 2,000 pages. Just thinking about it makes my head hurt. Being Tallis, finding oneself consumed by the big questions, is hard work.

“The big story is, what is a human being? If we don't define ourselves supernaturally, then what kind of creatures are we? And how can we reconcile the profound joy we can feel with the awfulness of our mortality and the suffering that goes with it?”

I don't know and nor, yet, does he. But it is reassuring to know that his intellect is now combined with a more relaxed approach. “Your diagnosis is quite right. I have lightened up,” he says cheerfully. “I used to think that difficulty was the mark of profundity. Now I think that accessibility is.” So when he does find those answers, we will all be able to understand - and enjoy reading about them.

The Kingdom of Infinite Space: A Fantastical Journey Around Your Head, by Raymond Tallis, is available from Atlantic Books on April 1 at £19.99 and from Times BooksFirst at £17.99. 0870 1608080; timesonline.co.uk/booksfirst, free p&p.

Tallis on what makes us human

KISSING

Kissing is both a biological and a social scandal. Biologically, it is a waste of time. And socially it is deeply problematic. There are, of course, grades of kissing: mouth to epithelium (hand, top of head, cheek); closed-mouthed lip to lip; open-mouthed lip to lip; and the full French.

Locating a kiss at the right place in the spectrum can cause trouble. As I write, a vicar and school governor who kissed a prizewinning girl on the cheek when he handed over her prize at the prizegiving ceremony has been referred to three separate disciplinary bodies for investigation, evaluation and appraisal of his possibly inappropriate behaviour. The matter has evoked national comment.

A full-on kiss is massively, and deliciously, transgressive, not only because it breaks the elementary rules of hygiene. Under normal circumstances, the intercourse between heads, whereby each tastes something of what is in the other's head, is quite circumspect: verbal or visual, exchanges of sentences and or of facial expressions and glances. The kiss bypasses all this. It is a regression to the carnal directness of infancy. The mouth as an organ of speech, of relatively remote communication about things brought into play through their general or abstract properties, is returned to its origin as a tactile organ.

This is not, of course, the whole story. We remain, as we kiss, knowing animals, aware of the place we have come from and of the distance we have travelled. This is why the kiss, for all that it makes direct contact with the other as organism, still retains the ghost of a dialogue that acknowledges the other for what she is as a person. And it is why those who kiss often close their eyes. To quote W.H.Auden:

Lovers, approaching to kiss

instinctively shut their eyes

before their faces

can be reduced to

anatomical data.

So there we have it. A kiss casting light on our carnal being, on the interweaving of lives, and on joy.

LAUGHING

We are inclined to laugh when we are in the presence of someone else who is laughing or even when we listen to canned laughter. Just how infectious laughter is was illustrated by an episode in 1962 in a girls' boarding school, where, for a period of six weeks, the school was forced to close. It began with three girls who were stricken with bouts of uncontrolled laughter for hours on end. Eventually nearly half the 159 school boarders were affected, laughing for up to 16 days at a time. The school was closed and the children sent home but this resulted in further spread of the condition throughout communities and to other schools. Attempts to reopen the school were disastrous. In the two-year period that the gelastic epidemic lasted, 14 boarding schools and entire villages and towns were affected round the eastern shore of Lake Victoria. Laughter under such circumstances is no laughing matter.

Humans are not only the sole creatures who laugh; they are also the only creatures who reflect on laughter. They plot ways of bringing it about, think about what is funny and what is not, and acquire a livelihood through an entertainment industry devoted above all to sex and laughter, and often the two together.

The saddest laughter is the laughter of the mad. Inappropriate laughter is an early sign of psychosis. Under normal circumstances, we laugh 30 times more frequently when we are with others than when we are alone; for the madman this is reversed. He does not laugh with the world but at it; and the world rewards this by laughing at him.

YAWNING

Yawning, like laughter, is infectious only more so. Stamp collectors may like to know the meta-yawning fact that 50 per cent of people will yawn within five minutes of seeing someone else yawn. Even thinking about yawning is enough. It is a safe bet that, while you might have read through the section on laughter entirely po-faced, you will have yawned by the time you have reached this sentence.

CRYING

Tears illustrate something that goes to the very heart of what it is to be a human. While other mammals may lachrymate in response to extreme pain, as well as to launder the eye, only humans cry or weep as an emotional reaction. In such cases, the lachrymal glands contract to increase their output, and while some of the output will take the usual exit into the nose, stirring up the mucus and making it run, the ducts may be overwhelmed and tears trickle down the cheeks. In addition, there may be sobbing: contortion of the face, with convulsive expiration and non-linguistic vocalisation.

Emotional tears (for example, those provoked by Tennyson's line “Dear as remembered kisses after death”) are richer in manganese and protein than those prompted by pain elsewhere in the body. Tears may be judged by the quality of the emotional thoughts that lie behind them: self-pity, pity for others, sympathy, compassion. At the bottom of the scale, we have the tears of a child angry that another has been preferred over himself; or that an ice cream has been denied him. They may be accompanied by physical actions such as the gnashing of teeth that bite themselves for the want of an accessible or appropriate external target.

There are tears of grief that we all respect because they draw upon the sources of suffering to which we are all vulnerable. These are tears we want to share. Indeed, we worry for the bereaved one who does not weep: we fear the effect of all those undischarged emotions.

VOMITING

There can be few experiences so all-consuming as vomiting. No orgasm, symphony or wartime briefing can command such total attention. Your body has you in its entire grip. There is a sense of helplessness as you are frogmarched through a series of stages; nausea that grows in intensity with sweating, pallor and the copious salivation as the harbingers of the gathering storm; past the preliminary heaves and retches; and then on to the gross and utterly engrossing climax. There may be a minute or two for you to prepare, once the ordeal seems inevitable: time to take off some clothes, to kneel down by the toilet bowl, and to take a few deep breaths, and thus stockpile some oxygen, for who knows when you will next be able to breathe freely without the danger of inhaling vomit.

There is a kind of terror in vomiting: it is a shouted reminder that we are embodied in an organism that has its own agenda, and that agenda might not include breathing for the present. The unpleasantness is compounded when the stuff runs down the nasopharynx and the delicate membranes are scorched by half-digested nutriment stewed in hydrochloric acid.

Here's what you would have to do if vomiting did not happen involuntarily. Take a deep breath, close your glottis (the opening to the airway) and open your upper oesophageal sphincter (the opening to the gullet). Elevate your soft palate to try to stop the vomitus from flooding up the back of your nose. Pull your respiratory diaphragm down sharply to create a negative pressure in the chest, which will cause the oesophagus and the sphincter between it and the stomach to open widely. At the same time, contract the muscles of the abdominal wall, squeezing the contents of the stomach which will shoot up the gullet via the mouth to the alarmed, fascinated and nauseated outside world.