Between those who march and those who instinctively distrust mass protest, there lies the great supine majority.
We like the idea that others are prepared to put up with the cold, the discomfort and the possibility of being cracked over the head by a police baton, and support the idea of protest, safely, from our sofas. When things get out of hand, most adopt the Cameron position: peaceful demonstration is good, violence is bad.
Amid the sorrowful scolding of those student protesters who have, obviously and on camera, behaved in an anti-social way, certain obvious points are forgotten.
The ingredients of legitimate protest - frustration, alienation, boredom, and the desire for change - are precisely what also caused the violence.
One student, Charlie Gilmour, persecuted in the Press because his parents are famous, has tried to explain that he was carried away in the spirit of the moment.
Anyone who has been in a volatile, clumsily policed crowd, whether at a demo or a football match, will know how quickly mass anger can boil over. Police get carried away too, and the downward tumble into fighting begins. These things tend to be forgotten by the reasonable non-marchers, sitting at home - except, that is, when they are in a historical context. Similar scenes of street-fighting were commonplace 40 or so years ago.
At the time, the anti-Vietnam war demonstrators caused outrage.
There was mockery of some of the protesters' sillier ideas: at one point, during the Grosvenor Square demo, there was serious discussion as to whether that symbol of Yankee imperialism, the Playboy Club, should be attacked.
"A kind of contagious frenzy gripped many student bodies," wrote Bernard Levin with some distaste in The Pendulum Years, adding wearily that there was nothing new in the young going too far. Because it was written by the grown-ups, "recorded history . . . had been in this respect one long catalogue of the crimes and follies of youth".
Not anymore. When the grizzled veterans of sixties protest look back, it is with nostalgia and fondness.
What happened then is no longer hooliganism and unkindness to police horses, but a laudable expression of radicalism and citizen power which later played an important part in politics and society generally. The revolting students were not yobs, after all; they were heroes.
When I sat my finals at university, the undergraduate next to me stood up, shouted words to the effect that it was all nonsense, tore up his question paper, and walked out. It was John Barker, later of the Angry Brigade, already gaining a stellar reputation for taking protest further than most would dare. I have a sense that, with or without his degree, he got more out of university than I did.
Only time will tell whether the marches and riots of 2010 will prove to be as significant as those of 1968 and beyond, but already it is clear that they represent a change in the political weather.
Authority figures and liberal hand-wringers, neatly represented by Cameron and Clegg, have tried unsuccessfully to appeal to youth. The media has played its part - who could forget the embarrassing, young persons' Question Time during the election campaign?
The students have responded with commendable bloody-mindedness, refusing to do politics in the way suggested by the grown-ups. They have overturned the assumption that theirs is a cosseted generation who can be kept quiet with a Facebook page, some Twitter, and The X Factor.
It is all rather awkward for the adult world. Violence, by protesters or police, is bad - but the spirit behind the new wave of activism gives hope for the future.