BOB DYLAN recorded Like a Rolling Stone 40 years ago this summer, and the Dylan debate has been gathering moss more or less continuously ever since. Does Dylan deserve his reputation as a musical poet of genius? Those who would deny him the title include such luminaries as Stephen Spender and Anthony Burgess. Those who consider Dylan to be among the greatest practitioners of the English language include the critic Christopher Ricks, the poet Allen Ginsberg, the film-maker Martin Scorsese and, as it happens, me.

Dylan himself is agnostic on the subject (or pretends to be). One delightful moment in Scorsese’s superb new three-and-a-half-hour documentary of his hero comes when Dylan recalls an adolescent crush on two girls: “Those two girls, by the way, brought out the poet in me.” Then he grins with sly self-mockery, instantly sending up the entire Dylanology industry. The “by the way” is critical.

Dylan’s place in the poetic pantheon will be disputed for ever, but what is certain — and what becomes even clearer in Scorsese’s adoring bio-pic — is that this has been an exemplary artistic life, an object lesson in creative brilliance. Dylan set words to music in a way that no one had done before. He refused to be pigeon-holed by the folkies, the protesters or the rockers. He borrowed and synthesised from the literary, artistic and actual worlds like a musical magpie, and he skilfully evolved his own mystique. And he kept going, even when his listeners booed or complained or, like the enraged Pete Seeger in 1965, threatened to chop off his sound cable with a hatchet at a folk festival in Newport because he had defected to electric sound. At a British concert, we see a furious folkie leaping to his feet and shouting “Judas!” Dylan is defiant: “You’re a liar . . . Play it f***ing loud,” he instructs the band.

Dylan understood and took inspiration from high and low culture, old and new, in a way that makes the Beatles look naive and the Rolling Stones positively facile. He understood the canon, and his potential place in it, like no other modern singer-songwriter.

In 1960 Robert Zimmerman, a gawky Jewish boy from Minnesota, hitch-hiked to New York City. He came to join the burgeoning folk music circuit, but he also came to read, hunkered down on the sofas of his bookish new friends in Greenwich Village. “I read all of Lord Byron’s Don Juan and concentrated fully from start to finish,” he wrote later. “Also Coleridge’s Kubla Khan. I began cramming my brain with all kinds of deep poems. It seemed like I’d been pulling an empty wagon for a long time and now I was beginning to fill it up and would have to pull harder. I felt like I was coming out of the back pasture.”

Gogol, Balzac, Hugo, Dickens, Thucydides (“a narrative which would give you chills”) Tennessee Williams, Bertold Brecht, Jules Verne, H.G. Wells: all were piled into the wagon, alongside the music of Woody Guthrie and Hank Williams, and the films of Marlon Brando and James Dean. He spent nights studying the American Civil War at New York public library and consuming newspapers: “What was swinging, topical, up to date for me was stuff like the Titanic sinking, the Galveston flood, John Henry driving steel . . . this was the news that I considered, followed and kept tabs on.”

Heck, he even managed to find musical inspiration in Clausewitz. News junkie, bookworm, opportunist culture vulture, Dylan raked up Rubens, Rimbaud, Roy Orbison and the Police Gazette and from this came the cascading imagery, the balladeer’s repertoire, the skewed lyrical improvisation and the (briefly) urgent politics. Here was a man of feral beauty with a strange, strangulated voice, singing new truths to ancient lays.

Dylan’s self-description is closest: “a musical expeditionary”. He was ruthless in taking what he wanted on his forays and, like a true artist, acutely restless. He made and remade himself, taking his cue from Rimbaud’s je est un autre: “When I read those words a bell went off . . . I wished someone would have mentioned that earlier.”

Counterbalancing the passion and the showmanship was a vein of self-irony never far below the surface. The Scorsese film shows footage from a press conference, where Dylan is asked how many other protest singers there are like him in the US. He deadpans: “Right now, 136.” “How can you be so precise?” wonders the journalist. “Well, maybe it’s 142,” says Dylan.

Dylan made a pact with his muse: she gave him everything, but he worked hard for her, and continued to work, even when the magician’s touch had faded. Since 1988, he has played 1,700 concerts, and when the critics carped, he played the music louder, because that was the deal. “I made a bargain with it a long time ago,” he once said. “And I’m holding up my end.”

“Come writers and critics who prophesy with your pen,” he invited. So, here goes: I predict that 40 years from now Like a Rolling Stone will still be voted the greatest song of all time by Rolling Stone Magazine; that we will still be arguing over whether Dylan wrote anything of comparable brilliance after 1966 to what had come before. I also prophesy that Professor Ricks will have written an Empsonian exegesis on Dylan with more footnotes than text; and that Dylan will have inspired more PhD theses than the rest of pop music put together.

“Don’t criticise what you can’t understand,” he said, and few would claim to understand Dylan completely: he remains the mystery tramp. Call him a poet or a pop star, Dylan is the master of performed literature, combining profundity with extraordinary simplicity, sung to melodies that anyone can hum, and few can forget.

With what was surely his most knowing grin, Dylan once observed: “All I’d ever done was sing songs that were dead straight and expressed powerful new realities.” I can think of few better summations of the artist’s calling: the mantra of a musical expeditionary, the boy from the back pasture, forever young.

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