But there is one scene, one cameo performance which, above all the others, captured the occasion for ever. It was the great toilet cleaning scene featuring one flawlessly stoned freak.
A benign workman is sucking out and disinfecting a row of temporary loos - Portosans. He clearly likes his work and is well- disposed to the 400,000 stoned dreamers lurching about in the mud and the music. As he finishes the job and coils up his fat suction tube, this smiling, blissed-out freak - in the amiable, Sixties sense of the word - floats out of one of the cubicles. Thin and bushy, he is the perfect specimen of the genus. He is bearing a pipe and looks transcendentally bewildered. He offers a smoke to the camera crew. They decline and ask him about the sanitation.
'Wow, it beats the woods,' he says. 'You makin' a movie? What's it gonna be called?'
'Oh, far out,' says the freak, and drifts off.
The face of this freak still haunts me. It was a face possessed of inane sweetness and narcotic innocence, the type of innocence, as T S Eliot once said of a poem of Blake's, against which the world rebels. That daft, grinning face dissolved in an instant all the big question marks that were already hanging over the decaying Sixties.
OK, the Vietnam war was a more complex matter than the draft- dodgers and the protesters were prepared to acknowledge. OK, wicked capitalism didn't turn out to be so bad after all. OK, drugs weren't quite the friendly affair they appeared to be in the heaving swamp of Max Yasgur's farm.
But still, confronted with the exquisite acceptance of that 'Oh, far out', only a fool would deny that, for one brief, shining moment in history, the freaks had a point.
The point was that the unstoned future didn't look so bright. There was a cold nuclear war that appeared ready to foreclose on all futures in a flash and there was a hot, dirty conventional war that lacked all the noble, simple rhetoric that young death demands.
'Don't ask me, I don't give a damn,' they all sang at Woodstock, 'next stop is Vietnam.' And then the perfect chorus, lampooning the discredited dignities of a war made pointless by the incomprehension of the young: 'No time to wonder why. Whoopee, we're all going to die]' In 1969 the Seventies were generally assumed to have been cancelled.
As well as this fatalism there was also a destabilising sense that affluence and freedom were not so hard after all. The good life no longer seemed to depend on conformity, the rabid rationalisms of the Rand Corporation or the furious paranoia of the American military. Suddenly you didn't have to work the system, you didn't even have to work, you could just hang out and take in the music. Somebody, somewhere paid, and nobody had to get napalmed.
From the perspective of the Nineties, this might be said to have been a convenient system of illusions, the phoney justifications of spoilt rich kids, insulated from political and economic reality. And so it was. But, from then, it felt as true as anything ever can. It seemed that descending on a field to worship at the feet of (mainly) cheap, though potent, musicians made perfect sense.
This was a new heaven and a new earth. Woodstock, as the people in the film repeatedly insist, was a city: a uniquely peaceful American city requiring neither cops nor guns.
And later it became a whole country, the Woodstock Nation. American utopianism had turned itself inside out - the city of God was a swamp, and 'making it' turned out to involve a flamboyant, muddy display of poverty, not a clean, disciplined acquisition of wealth. Dionysus had succeeded Ford. Walt Whitman had defeated Lyndon Johnson. God had conquered Mammon.
'The time has come,' said the bearded, beaded Indian guru to the crowd, 'for America to help the whole world with spirituality also.'
'People that are nowhere,' said the most soberly perceptive of the hippie interviewees, 'are coming here because there's people that they think are somewhere.'
All of which is to say that last weekend's Woodstock II never stood a chance. The freak's face, seen in grainy close-up 25 years later, said it all: never such innocence again.
Today's quasi-freak wouldn't think the Portosan was better than the woods, he would think it was an outrage, bearable for weekend maybe, but scarcely a harbinger of a new world order. He would vent his anguish on video, not film. And, naturally, cellular technology would mean that he wouldn't have to queue for the payphones, as they did in one of the movie's other great scenes, but would simply dial into the Net.
The idea - the illusion - behind Woodstock II was, I suppose, continuity, an attempt to say that rock culture and its libertarian meanings endure intact. An approximate C-major thrashed out on a Gibson Les Paul means as much now as it did in '69. We did it once, we can do it again.
But, of course, we can't. And for the simple reason that we have, in the West at least, inherited the new world order promised by Woodstock I. Perhaps the funky pastoralism has gone, but being free, being yourself, letting it all hang out, is now little more than the code of the average Harvard business graduate. The hot, sweet hit of rock is controlled and marketed by the same system for whom the boys were said to be fighting in South-east Asia. Woodstock convinced everybody except the rednecks. We bought the package. But it didn't get us back to the garden, it got us political correctness and the reflex, bumbling 'humanity' of the oh-so-hip American entertainment industry. If Woodstock II really wanted to take on the system, it would mean taking on Woodstock I.
But it couldn't do that, it could only go for a repro-festival, a feeble memorial to the original. 'It wouldn't be the same,' said all the veterans who decided not to go this time round. Quite right, how could it be? The magic of authenticity has been lost. The mud would be just mud and the toilets just toilets, there could be no grand vision to blind you to the mess, the suffering of it all could not be redeemed by its significance. It wouldn't be the same, it would be merely real.
Then the smart, younger commentators come along to twist the knife and claim it was just like this the first time round. Here is William Cash in the Daily Mail writing from Woodstock II about Woodstock I: 'Those who hailed it as the flowering moment of the Sixties were probably not there. Those who were there know it was a form of social torture on a scale probably not seen before this century.'
Dream on, William, or rather, see the movie. There were no dandyish murmurings about 'social torture', there was only loopy idealism. And, embarrassingly for the ironists of post-modernism, those freaks meant all that stuff. Woodstocks I and II looked the same - music, mud, lots of people - but context is all. And if the young of the Nineties find that hard to admit, it is because their context looks pretty boring in comparison.
Imagine freaking out because the ozone layer looked a bit iffy or because Bill Clinton invaded Haiti - it just doesn't work. Certainly Live Aid relit some of the fire in the Eighties. But that wasn't about a new world, it was about an issue, and you could almost hear the rock business sighing with relief that it was being offered the slender possibility of meaning again.
Woodstock, in short, was a one- off. It had no future because it was inspired by dissent, and once America really did become Woodstock Nation - leaving aside the mud and some of the mysticism - there was nothing to dissent from. To a surprising extent the freaks got what they prayed for - a looser, slicker, hippier society. The hippies now run the computer corporations, some guy who choked on a joint is in the White House and 'wow]' is a word in a toothpaste ad. If it still doesn't feel right, then the freaks have only themselves to blame.