When Bob Dylan materialises in the darkness on the stage of the Clyde Auditorium on Monday night, some people will be keeping count.

I can save them the bother. The first night in Glasgow will be show 2557 in the ritual they call the Never Ending Tour. For his part, the exasperated artist refuses to accept that such a thing even exists.

His struggle against what fans know as the NET has been unavailing for the best part of a quarter of a century. The legend arose from a single casual answer Dylan gave to a journalist from Q magazine in October 1989. One tour had all but run straight into the next, it was observed. "Oh, it's all the same tour," the artist replied. "The Never Ending Tour," declared the ingenious interviewer before proceeding to put the words into his subject's mouth. So it began.

Dylan has tried to kill the notion time and again. In 1993, his sleeve notes to the World Gone Wrong album warned fans to avoid becoming "bewildered by the Never Ending Tour chatter". Over the years, journalists have been told repeatedly that performing is just what Dylan does: "a job, my trade". Some have been subjected to that familiar, inimitable sarcasm. "Does anybody ever call Henry Ford a Never Ending Car Builder?" Dylan demanded of a hapless Rolling Stone reporter in 2009. "Is Rupert Murdoch a Never Ending Media Tycoon? What about Donald Trump? Does anybody say he has a Never Ending Quest to build buildings?"

Less facetiously, Dylan observed in the same interview that Picasso was still painting in his nineties. Elsewhere, he has talked of Count Basie, Duke Ellington and all the generations of musicians, in every style, for whom the endless highway has been intrinsic to art, craft and business. His own schedule, 100 concerts a year at a rough average, is nothing special, he says. By some yardsticks, that's the simple truth. Well into his seventies BB King was still giving between 250 and 300 shows a year.

You could also note that for Dylan, as for most performers, a collapsing CD market has made touring rather more attractive than once it was. Since his recordings have only rarely been mistaken for hits - perhaps 100 million "units" sold, but spread over half a century - he rediscovered the joys of the concert stage sooner than most. The days when he could disappear from public view for eight long years, as he did after a motorcycle accident in the summer of 1966, are long gone.

None of that begins to explain the enduring myth of the NET. Unofficially, the records say that it began in Concord, California, on June 7, 1988 when Dylan and a stripped-to-simplicity three-piece band appeared in a half-empty arena called the Pavilion. The absence of fans was easy to explain. Dylan, like his reputation, was at a very low ebb. The 1980s had been unkind. His latest album, the just-released Down in the Groove, would be the second in two years to fail even to penetrate the US Top 50.

It was not just a matter of changing times. A few perverse fans will tell you still that Down in the Groove is "underrated". Factually, it amounts to barely 32 minutes of music and contains only three songs in Dylan's name, each of which you would struggle to recognise as the work of the greatest songwriter of his century. Later, the artist would admit to interviewers that he almost "packed it in" as the 1980s drew to a close, that he had "reached the end of the line".

After witnessing a show at the Wembley Arena late in 1987, John Peel had been scathing about Dylan's apparent contempt for his art and his audience. "Being an enigma at 20 is fun," the disc jockey had written, "being an enigma at 30 shows a lack of imagination, and being an enigma at Dylan's age is just plain daft ... From the moment the living legend took to the stage, it was evident that here was business he wanted accomplished with the minimum of effort." When Dylan appeared in Concord the following summer, it seemed he was making his last stand.

The Interstate 88 tour would begin as a hard slog by an artist in the face of critical derision and end, after 71 shows, with a triumphant four-night residency at Radio City Music Hall in New York during which every reviewer in town turned on a dime. "Extraordinary no-frills rock and roll," said one. The fact was that Dylan was rediscovering an interest in his own songs and in musical tradition. Somehow, as though from the depths, he was drawing energy from art.

That tale would do nicely for most musicians. In a proper Dylan legend, however, it is barely even a start. If you believe half the fabulous nonsense - and he was complicit in some of it - the singer didn't simply pull himself together before commencing Interstate 88. Instead, he experienced what some of the hardcore fans call, with little irony, an epiphany.

Specifically, on a foggy, windy night in Locarno in Switzerland, peering out at his audience on the Piazza Grande while hiding behind his backing singers, Dylan experienced a moment of reaffirmation. He later told the story to various journalists. To one he said: "It's almost like I heard it as a voice. It wasn't like it was me thinking it. 'I'm determined to stand whether God will deliver me or not.' And all of a sudden everything just exploded. It exploded every which way."

Afterwards, Dylan said he "sort of knew: I've got to go out and play these songs. That's just what I must do." The creative drought of the 1980s began to come to an end. His next album would be the thrilling Oh Mercy. At Concord, the NET would be inaugurated and would come to be treated, by some, as a continuing art project in its own right. In some corners of "Dylan studies", in fact, live performances are these days regarded as the keys to the songwriter's art.

But hold on. Bootlegs say that nothing "exploded" in Locarno or during the following concert in Paris. Peel's condemnation of a lazy, empty caricature of a living legend saw print after that "epiphany" in Switzerland, not before. Oh Mercy, fine as it remains, turned out to be another of many false dawns in Dylan's career between Infidels in 1983 and Time Out Of Mind in 1997. The unending tours varied markedly in quality.

Meanwhile, one fact glares at anyone who writes about this artist. Irrespective of any voice Dylan thought he heard in Locarno, seven years would elapse between the 1990 album Under the Red Sky and Time Out of Mind. In that period the reborn artist released not one new song. The touring continued relentlessly, a pattern of travel woven over and over on the face of the globe. But the NET, this art event, this disputed legend, did nothing for Dylan as a songwriter.

The fact that with Time Out Of Mind he commenced one of those second acts supposedly forbidden in American lives involves another fascinating story. Last year's Tempest album demonstrated that Dylan remains a potent writer and a formidable recording artist. But as a concert artist? Year after year people walk out of his shows. The complaint is not that he has rearranged favourite songs. The gripe is that the songs are unrecognisable in any form.

A Dylan show these days is pot luck. Two and a half thousand concerts, an untrained voice given insufficient rest, the ravages of recreation, age itself: even some dedicated fans have stopped asking why he does it, year upon year, and begun to question whether he should be singing live at all. Undaunted, NET veterans hold out the promise of the magical nights when everything comes together. The teenager hoping to hear the legend might end up with a different view.

But he's Bob Dylan. Advice has always been wasted on this artist. Hindsight has tended to show that he knows better than his critics and fans. Some of them need the myth of the Never Ending Tour more than they or he care to admit. Even if the voice has all but gone and the songs have been mangled to suit that lingering growl, such people cannot imagine the world without him.

Now that's art. Like him, it endures.

Ian Bell is the author of a two-volume biographical study, Once Upon A Time: The Lives Of Bob Dylan and Time Out Of Mind: The Lives Of Bob Dylan. Bob Dylan plays Clyde Auditorium, Glasgow on Monday, Tuesday and Wednesday.