The word "revolutionary" is so frequently used today, its coin debased by journalistic clichés and marketing slogans, that we've lost track of its true meaning. On Friday, we learned what the real thing feels like.
It's a joyous moment, one we should never forget no matter what follows. The unstoppable force of human resilience met the immovable object of one of the world's most entrenched autocrats and, after 18 days of obstinate determination, the pharaoh dissolved into the sand.
Nobody can take this moment from the Egyptians. They've done it themselves, without violence or fanaticism, without celebrity leaders or overweening ideologies, without any help or funding or guidance from outside - certainly, to our shame, without the backing of our government.
Friday's freedom-mad street dance in Cairo was watched by the world, but authorities in Beijing, Tripoli, Damascus and especially Tehran did everything they could to prevent their citizens from watching it: National TV in those countries was notably quiet; Iran scrambled the broadcast signal of the Persian-language BBC in advance of its own Monday demonstrations. And no wonder: If it can happen in Egypt, it can happen anywhere.
"To dare: That is the whole secret of revolutions." The words of Antoine de Saint-Just, who was part of the crowd that drove another tyrant out of power, are becoming part of our currency again. But as the euphoria fades to exhausted self-scrutiny, it's not those words we'll remember but rather what Saint-Just was doing when he uttered them: making use, with fellow post-revolutionary leader Maximilien Robespierre, of the policy instrument known as the guillotine - and following that other great revolutionary tradition in which the glorious popular uprising leads to chaos, extremism and, sometimes, more tyranny.
Revolutions tend to defy hopes and expectations, of both the men on the balcony and the people in the square.
Sometimes, the crowd's grab for popular power is seized by ideologues or confiscated by those who control the military, or caught in a violent oscillation between the two. These scenarios, in their extreme versions, don't appear likely in Egypt, but all possibilities are present.
The first modern revolution, in England in 1640, set the template: a joyous uprising against the king, the first popular trial and execution of an autocrat, but then Oliver Cromwell's bloody republic, followed by the restoration of the monarchy. Then, after a surprisingly successful American example in 1776, came France in 1789: revolution, popular sovereignty, then the Terror, the authoritarianism of the Directory, then Napoleon's coup.
Many of the multihued uprisings of recent decades have suffered this fate. In some cases, the seizure is lengthy. It's been many decades since the popular aspirations of Cuban and Iranian revolutions were hijacked by shallow autocrats; but, beneath the surface, their original aspirations remain frozen in amber.
And that's what we must remember: Even when they're sidetracked or seized, the seeds planted by a democratic revolution remain in the ground.
My favourite statement about revolutions is Karl Marx's pamphlet The Eighteenth Brumaire of Louis Bonaparte, in which he looked back bitterly on the uprisings that swept across Europe in 1848. His followers, after all, were meant to have been the ones who'd seize those popular revolutions and use them to further their own ideological ends. Instead, the people wanted the old bad guys gone but weren't so interested in the dictatorship of the proletariat. They were just democrats, not utopians - much like the kids in Cairo on Friday.
Marx accused the peasants and merchants in the streets of misunderstanding themselves. But they understood themselves all too well. "Men make their own history," Marx concluded, "but they do not make it as they please; they do not make it under self-selected circumstances." And that's just as well.
The joy of revolutions is that they make ordinary life interesting. Suddenly, the streets glow with importance; anything seems possible. But this is also their great flaw. For revolutions are about the state, and we generally don't want the state to be interesting. We want our lives to be interesting, and the state to be the safe and neutral background against which our lives unfold. When revolutions really succeed, the state is able to fade into the background - perhaps governed by those who are weak or disagreeable or incompetent or somewhat corrupt, but in an ordinary and banal way, until the next election.
I've always thought that the great thinker of revolutions wasn't Marx but his fellow German, Max Weber, who saw history not as a march toward utopia but as a succession of moments of "charismatic authority" - the exciting leader who seizes the day after the revolution, the bold new set of governing ideas - followed by periods of "routinization" and bureaucratic management of the formerly charismatic ideas.
The sooner that routinization of charismatic authority happens, the better. Egyptians will soon pass through a period of charisma, and its opposite, and possibly further struggles, but what they've won is the right to be quietly ordinary.