Umberto Eco is a rare thing: a high priest of semiotics who sells novels by the million. He also eats a lot.

Odd name, Umberto Eco, almost too good to be true. Umberto evokes, perhaps, Humbert Humbert, hero of Lolita. And Eco - well, it's Italian for echo, an absurdly perfect name for a writer who deals with the outer limits of memory and representation. But odd or not, that is what he is called, Humbert Echo, Post-Modern superstar, best-selling skypilot of the Zeitgeist.

Except that it is not his name. His paternal grandfather was a foundling. In Italy such children are often left with the church and given names to indicate their status - Esposito, exposed, is quite common. For years Eco assumed his name was given by an official with a slightly odd sense of humour.

Then he was contacted by a woman in Naples with the same name. Perhaps this official had also worked in Naples. But then, a few weeks ago, a friend researching in the Vatican library discovered that the Jesuits sometimes gave foundlings the title Ex Caelis Oblatus - offered by the heavens - or Eco for short. Umberto, the explorer of signs, is an acrostic.

And, these days, he is a very fat acrostic. Through the years of his fame - to be exact, 15 years since The Name of the Rose was first published - he has spoken in interviews of his battle with his paunch. Now, at 63, he has clearly lost. The belly swells dauntingly from the confines of his dark suit, turning the voluminous shirt into a tense membrane. In his Chelsea hotel he rolls in the armchair and draws steadily on a succession of Philip Morris Super Lights. He is a big, roly-poly, grey-bearded, literary godfather, slightly frightening and with, I suddenly notice, peculiarly small feet.

His English is good but difficult. For some time I am baffled by the phrase "new roses", assuming this must be a fancy way of speaking of books written after The Name of the Rose. Then, suddenly, I realise he was saying "neurosis" and 10 minutes of dense exposition suddenly falls into place.

He is in town publicising his third novel, The Island of the Day Before. It is a less traumatic experience than the publication of his second.

"The crucial point", he explains, "was to shift from the first to the second novel. The first novel could have been a unique case. To write a second was to prove myself. After that it was irrelevant whether I wrote a third or a 10th novel."

He takes his time. His second novel, Foucault's Pendulum, took eight years and The Island took six. When they are finished he feels like an orphan and wants to start another at once.

"The wish to start a new one is very strong, just to regain the pleasure. But I must resist. I don't want to write a novel per year. I know that I need a break of one or two years. So maybe I invent some new, urgent activity so I don't fall into the trap of starting a new novel."

There is plenty of scope for urgent activity. He is still Professor of Semiotics at Bologna University, the arcane job that he was doing when he sprung The Name of the Rose on the world. That novel sold 10 million copies worldwide and transformed the fortunes of Eco.

"When it started", he recalls, "it was a game. I thought it was nice to write a criminal novel in the common sense of the term. Only later I understood that I was putting in all this philosophy and history and the social life of the Middle Ages. It started as a 200-page story and it ended up as 500 pages. Certainly I have always felt that a novel - be it Balzac, Stendhal or Dickens - must be a page-turner. And I did use all these familiar narrative mechanisms - the ghost, the secret and so on - but I knew that I was using them ironically. But, for certain readers, it was not so ironic."

This is the heart of the Eco oddity. Here is this highbrow academic, high priest of semiotics - an arcane, difficult and often wilfully incomprehensible discipline - who decides to play sophisticated games with the novel form. It sounds like a recipe for an obscure succes d'estime, a few hundred copies at the most. But no, Eco sells millions because his smart literary games work in a completely unsmart way. The Name of the Rose flows and thrills as surely as a Dick Francis.

This combination of smartness and popularity, plus his mastery of the fashionable discipline of semiotics, has made him a Post-Modernist icon, a hero of the globalised culture-surfers. Here is a man who can see the truth about blue jeans as surely as he can decipher the nuances of medieval nominalism. Unsurprisingly, Post-Modern is a label he accepts.

"It is a notion so big that we are always going backwards to find its beginning. The definition I give comes from Nietzsche. He said that our era is overwhelmed by historical consciousness; we know too much about our past, we are unable to be original. Post-Modernism was the way in which our culture has escaped this sort of new roses... " - sorry, neurosis - "... it says that we cannot write in an innocent way, ignoring previous literature. But we can do it through irony. It's a way of saying we know we are saying something that has already been said. If that is Post-Modernism - OK, I accept."

He does not necessarily think this is a new literary posture. Don Quixote, he says, was a novel about previous novels. Indeed, he believes the emotional remoteness of the writer implied by Post-Modernism is, in fact, a timeless literary truism. He does not believe spontaneous, emotional outpourings can ever be literature.

"If I had written a page of personal diary, it would be pathetic, not sublime. I read a wonderful passage from the second century: 'Pay attention, not all the sublime is pathetic and not all the pathetic is necessarily sublime.'

"When the poet is in love, he is incapable of writing poetry on love. He has to write when he remembers that he was in love. Poetry is not a matter of feelings, it is a matter of language. It is language which creates feelings. The love poetry that fascinates is the most artificial. Shakespeare's Dark Lady, Dante's Beatrice, Petrarch's Laura - those women had disappeared long before the poems were written. A young student who writes his love poems because he is raging for some girl is not a poet."

But what, if it is not love, is Eco distanced from? What is he recollecting in literary tranquillity? The question seems to irritate him. I suspect he has detected that I am not enthusiastic about his writing. He rolls forward and fixes my eyes somewhat threateningly.

"It is not my personal distance, it is the language's distance. Take this book... " - he points at a copy of The Island - "... it's about an unreachable island, an object of desire that is out of your grasp. In my life, like every human being, I have had that experience. There was something I couldn't reach or some place I couldn't be. I was unable to be there and it was out of my control. That's a normal human experience, but the way of transforming it into literature is to tell it in a baroque style... it's the language."

So what is literature for?

"It's a form of education of the reader. I educate him not to expect consolation from literature, only problems and questions. Probably, yes, this is the centre of my work, not to expect consolation, not to end the book very happy because they married and lived for ever. I think everybody writes literature in order to make a statement about life. My statement is not to expect consolation because life is not easy. This is not a personal discovery of mine."

So why read his books?

"There is the pleasure of the difficult experience. Musical compositions can be very sad - Chopin - but you have the pleasure of this sadness. The cheap consolation is: you will be happy. The higher consolation is the pleasure and recognition of your unhappiness, the pleasure of having recognised that fate, destiny and life are such as they are and so you reach a higher form of consciousness."

This, he says, is why he writes novels, to show his doubts. If he knew anything for sure, he would write it as philosophy.

The problem with all this is that, encyclopaedic as Eco's knowledge of literature is, there is something he doesn't quite get. Sure, the writer is remote, sure, art lets you down, but there is a poignancy in those truths that you simply never get from reading his novels. Nabokov or Beckett can make hot tears spurt with their explorations of the limits of art. But in Eco there is no pressure, it's all smartly put together to tell you nothing. The result is not a higher consciousness but complacency. That's the trouble with Post-Modernism, you can't really get involved. It's not done. The Island of the Day Before is clearly not worth reading by page 50. He wears his learning too heavily, he lacks wit.

But never mind, the Humbert Echo show rolls on. The interviews come and go. Much work is to be done at the department of semiotics at Bologna. Eco rolls out of his chair, shakes my hand and ambles off to consult his publicist. It is all such a wonderful, effortless, Post-Modern whiz. Italian without tears.

n 'The Island of the Day Before' by Umberto Eco, published by Secker & Warburg at pounds 16.99, is reviewed in tomorrow's Independent