Brian Eno is 'a mammal, a celebrity and a masturbator'. And his new diary could establish him as the avant-garde Alan Clark

n the back cover of A Year with Swollen Appendices, Brian Eno's diary of 1995, he supplies a handy guide to some of the things that he is: "a mammal, an Anglo-Saxon, an uncle, a celebrity, a masturbator". Further insight into the mind of the enigmatic 47- year-old more often referred to as musician, record producer and artist is furnished by his diary entry for 26 August of last year. "Pissed into an empty bottle so I could continue watching Monty Python and suddenly thought 'I've never tasted my own piss', so I drank a little. It looked just like Orvieto Classico and tasted of nearly nothing."

Bodily functions have not previously loomed large in the public perception of Brian Eno, but that's all going to change with the publication of this heroically indiscreet volume, which could well establish him as the avant-garde Alan Clark. "The most attractive thing about the diary form for me" Eno explains, "is that it gets rid of the idea of separating thought off from the rest of what you do. I kept trying to write a book of [he says disdainfully] 'my ideas' and it looked so dull even I couldn't be bothered to read it. Then I realised that this was because I don't have a linear argument to offer, just a series of observations and a way of training yourself to notice things."

The result of Eno's nightly endeavours is a compelling mishmash of random aphorisms ("cooking is a way of listening to the radio", "saying that cultural objects have value is like saying that telephones have conversations''), impassioned meditations on the Bosnian situation, and whimsical musings on such intimate ephemera as the fact that he rarely gets erections while in Ireland. Not for nothing does his cyber-guru friend and e-mail correspondent Stewart Brand describe him as a ''drifting clarifier''.

Given that Eno is most widely celebrated for his collaborative endeavours - much of the most socially acceptable work of Roxy Music, David Bowie, Talking Heads and U2 bears his hallmark - it is probably for its insights into the people he works with that this diary will be most prized. Readers may like to guess which member of U2 might have inspired the following observation: "Singers are like Arabs. They abhor a vacuum. And a vacuum is defined as 'when I'm not singing' " (clue: his name rhymes with mono).

Being a record producer, Eno insists, is "the best paid form of cowardice; producers often get praised but they have to do a really bad job for anyone to criticise them." If he chose to devote his time solely to record producing and gave up organising art exhibitions, making incendiary speeches at the Turner Prize and inventing new forms of computer-generated music, Eno reckons he could earn "five or eight million pounds a year". Had his accountants not persuaded him that, in this case, discretion was the better part of valour, he would have published a complete balance sheet of his income as one of his diary's swollen appendices. "You can talk about anything else," he says bemusedly, "your sex life, your medical history; but everyone is so circumspect about money".

What Eno's diary lacks fiscally it more than makes up for in psycho-sexual candour. "On the beach," he notes on 15 August, "watching topless French ladies with huge wobbling sousaphones of bumfat, wishing I could hear them fart." Didn't his wife Anthea (she runs Eno's business affairs) take exception? "When I first got the idea of publishing the diary, she had never read it. I said 'I don't know if I should show you this - I do go on about ladies' bottoms,' and she said, 'I don't mind, I've got one too'."

Eno's own bottom is seated as he speaks. It's a quiet Saturday morning at his studio in a quiet Notting Hill backwater, round the back of the Portobello Road. This beautiful airy room contains just about every possible means of amusing yourself - tools, musical instruments, computers, recording equipment, endless tapes and discs, a swing. Does Eno not find it hard to get down to work with all these different toys to play with? "You do have to be driven in some way to stop yourself from just frittering away your time and I suppose I'm driven by guilt. I've ended up in this quite privileged position, which so many people would love to be in, and I can't help wondering if I'm making the best of it."

Day-to-day existence Eno-style seems to be about as far from what anyone would call a normal life - having a job and coming home - as it is possible to get. How does he feel about that? "One of the things I don't dwell on in the book, because it would sound a bit self-pitying, is that being completely free to choose what to do is actually quite difficult: it can lead you to very depressive crises. If your time is structured you don't have this problem of 'what am I going to do today?' " Realising that this is quite a luxurious problem to have, Eno mocks himself for owning up to it: "I'm totally misunderstood [assumes whining artist tone] I've been waiting to tell someone about this for ages."

Judging by his diary's touching accounts of the Eno family dancing to doowop records together, Eno derives a good deal of inspiration from his two young daughters, Irial and Darla (pretty names, which their dad proudly confesses to having made up). Does he see creativity as a child-like state? "Absolutely. The way children learn is by pretending, imagining what it would be like to be in another situation, which is the essence of culture. There's this crazy supposition that at the age of 16 or 18 you should suddenly switch all that off because you now know what you're doing, but the people I find interesting are the ones who carry on playing that game."

What about his own background? You'd think Eno could only have been raised by a colony of free-thinking bohemian bee-keepers, but this was not the case. "My dad was a postman," Eno says fondly, "and my mother was a Belgian immigrant." How did they meet? "At the end of the war, when the Germans had left Belgium, they used to billet English soldiers with Flemish households, and while my dad was staying there he fell in love with this picture of a young girl, who turned out to be the family's daughter. She'd been in a German forced labour camp, building Heinkel bombers. When the war ended it took her ages to come back, she only weighed five stone and she had a one-year-old-daughter - the father had disappeared from the camp and was never seen again - but my dad was waiting for her. They got married a couple of years later, and she's lived in Woodbridge in Suffolk ever since."

That's a lovely story. "It's very romantic isn't it?" Eno grins winsomely. And any last remaining pretensions he might have to cold fish status are finally dispelled when conversation turns to the "very peculiar and eccentric" uncle he credits with a key role in his creative development. Though the man died four years ago (four years after his own father) Eno's normally calm and measured demeanour gives way to obvious emotional agitation as soon he starts talking about him. "His life story would really be worth going into," he enthuses. "He went to India with the hussars in the 1930s, but fell off his horse shortly after he got there, and was discharged with concussion. Then he spent six years hanging out with yogis and came back to Woodbridge," Eno laughs, "a changed man".

Eno's uncle worked as the town gardener, with a handy sideline in crockery repair, and used to slip his nephew tiny books (Eno demonstrates just how tiny with a simple hand gesture) from the Methuen World of Art series. It was while looking at two particular plates in one of these books, Piet Mondrian's two Boogie-Woogie paintings, "Broadway" and "Victory" that Eno realised he wanted to be an artist. "What knocked me out about them was something that I've been fascinated by ever since, which is how something so simple can produce such a strong effect. That economy has always been the thing with me. I want to do as little as possible for the maximum pay-off."

Inquisitive eyes meandering along Eno's video rack come to a screeching halt at the box marked "Mud-Wrestling". "I think people are so sensitive to what is erotic for them," Eno insists "precisely how much an eyebrow is raised or what word is used: they're probably more specific about that than anything else, other than food." Isn't that what makes those things exciting, the fact that people don't think about them? "I don't think people think about most things," he replies quietly, "but I do. I think, in general, thinking is a bloody good idea" - Eno laughs, and a shaft of sunlight glints, halo-like, off the top of his head - "and we don't do enough of it."

8 'A Year with Swollen Appendices' (Faber, pounds 9.99) is published tomorrow. It's Eno's birthday on Wednesday.