Howard Jacobson: How many roads must a man walk down before he develops a sense of humour?

Shown over two nights which I wish had been extended to 20, Scorsese's superb three and a half hour atomisation of Dylan's early career - it could not, I think, be called a celebration - was a belated reminder of what BBC2 is for. Three and a half hours! That's more television than I have watched without apoplexy in the past three and a half years.

It helped that the music was so good. Not just Dylan's, but that of those who made Dylan possible - Woody Guthrie, Pete Seeger, Johnny Cash, Odetta, Joan Baez ... The juggernaut of X-Factory pop has rolled over folk and blues, leaving them looking yellow at the edges even to those to whom pop is anathema. But there they were, the old sentimentalists, nothing like as naive or sanctimonious as one remembers, still fresh, in possession not only of better voices than we can muster, but better songs as well.

There wasn't just protest in the air in those days, there was lyricism. You turned up to a peace rally and you got Martin Luther King and Bob Dylan. Now you get Ken Livingstone and "Gorgeous" George, or on a good day Chomsky. You tell me who you'd rather listen to.

I'd forgotten how much I'd liked Bob Dylan's music. When this newspaper asked me to contribute to a Dylan fest I felt I had to refuse. What did I know? I'd never been to a Dylan concert - I don't as a matter of principle do concerts, though I have to say I can no longer remember what the principle is - and I'd never bought one of his records. But watching the Scorsese documentary made a liar of me.

In fact I did once own a Dylan LP. The Freewheelin' Bob Dylan, the one with "Oxford Town" and "Blowin' in the Wind" and "Don't Think Twice It's All Right". I bought it in Aden, in 1965, on the way to Australia by boat. Other passengers bought boxed-up duty free shirts which turned out to have no backs in them, but by the time you discovered the scam the boat had sailed. I had to wait until I got to Sydney to check whether Dylan was all there. Thereafter, for the three years I was away, I never stopped checking.

"It ain't no use to sit and wonder why, babe / It don't matter anyhow." For all the phoney folksy lonesome-roadiness of the idiom, that caught something about being 22 years of age and far from home.

As for the unexpected acerbity of its last goodbye - "I ain't sayin' you treated me unkind / You could have done better but I don't mind / You just kinda wasted my precious time" - it made me smile a melancholy smile I only now realise I have been smiling ever since. Not one of John Donne's great valedictions, not quite possessing Donne's invention or tenderness or even vitriol - these distinctions matter - but, when you take into consideration the music, who's complainin'?

But what a puzzle it is that a man can write that, can balance with such melodious aplomb the youthful itch against the youthful self-importance, can manage the exquisite wryness of it, and not have a sense of humour. I don't mean not much of a sense of humour. I don't mean less than you would have expected. I mean none. Nada. Zilch.

At periods throughout these masterful three and a half hours Dylan as he now is turned up in evasive three-quarter profile to comment (or not) on what we'd seen, a man in his 60s, devastatingly good-looking, his wire-hair powdered with lead pellets, his face so grooved and stubbled it looked as though a lawnmower with a broken blade had been over him in the night, and not a trace about his mouth or in the furthest reaches of his eyes indicative of mirth in any of its varieties. Not a trace of very much else, to be frank. Not a dead man, not a man on the dark side of the road, gone off, gone missing, gone somewhere else, but a man who had never been there at all. And certainly not a man for whom words are a natural medium.

Taciturnity in an artist is inexplicable to me. Sure, you get asked some stupid questions. And sure, it isn't your job, outside your art, to answer any questions at all. The work must do the talking. But you would think that just occasionally a passion for articulation would overcome all other considerations. The words, despite you, despite your hoarding of them, having their own way.

This conundrum - who the hell was Bob Dylan? - powered Scorsese's film. An opportunist, a magpie, a shaman, a holy spirit? "He was possessed," Liam Clancy affirmed with the authority of one who'd observed him closely. "It wasn't really necessary for him to be a definitive person. He was a receiver."

And of course, they say, a visionary. I bracket visionaries with experimentalists myself. Call a writer experimental and it means the experiment has failed; call him a visionary and it means he hasn't actually seen anything. "I've been out in front of a dozen dead oceans / I've been ten thousand miles in the mouth of a graveyard..." Sure you have, Bob. Sure you have. The mystery is, that for all the unmeaning, "A Hard Rain" is a song that haunts you.

An undefinitive person, then. Someone without a self. Like Keats's "chameleon poet". It makes one uncomfortable but that might simply be the way of it. Sometimes, something like genius takes possession of someone who isn't, passes right through him, and never touches the sides.