1 May is likely to remain the most convenient shorthand for the radical change we are now enjoying

History returned with the rain last week. After an interminable drought, it suddenly seemed as if the whole country was out and dancing in the downpour. On television and in newspapers there was a veritable monsoon of the "historic" and the "history-making". And on Tuesday, as if to reassure us that this wasn't one of those tantalising solitary cloudbursts, the kind that coax a perfume of dampness from the earth and then pass on, Gordon Brown announced that he was handing over control of interest rates to the Bank of England, allowing journalists and reporters to unpack the historic adjectives again.

It is all very invigorating after such a long dry season but it does raise the question of how exactly we decide what an historic moment is. Undoubtedly the early hours of Friday morning felt historic - so many announcements that called for a toast, the rosy blush of Not-Conservatism spreading across the map, the pallor of defeat on all those over-familiar faces. And the sensation of having experienced a cardinal moment in the national storyline strengthened over the weekend. The sense of astonishment, we learned, went to the very top - Tony Blair, hoping for a majority of around 40, apparently watched open-mouthed as the results came in, muttering the traditional incantation of those who have been ambushed by the future - "this isn't happening". It was, and just as surely as he had been elected to Number 10, the night on which the hope became reality has now been admitted to the select club of "historic elections", fit to be referred to alongside 1945 and 1832.

For both Blair and us, 1 May is likely to remain the most convenient shorthand for the radical change we are now enjoying (even those dismayed by the result will probably admit to feeling a secret sense of refreshment, the childhood pleasure of opening a new exercise book). But its claims to be the historic moment, the very pivot on which history turned, are a bit more debatable. As a decisive moment it certainly seems to satisfy some of the obvious criteria - the presence of a plausible alternative (the very ballot form reinforces the sense of other possibilities trembling on the brink of existence) and the feeling that the flow of events has been determined by human agency.

The public will make itself felt, is how it is conventionally put - a phrase which appeals to our desire to have some control over our lives. But in another light you might view the election very differently - as a result rather than a cause. It will have its own effects, naturally, because our system of government requires this public ratification of shifts that have already taken place as a ceremonial transition. But an election is as much an act of summation as it is of prediction, and in that sense it points back towards moments that may be less conspicuous, even more contingent, but have just as good a claim to momentousness. The fact that we didn't notice them at the time and played little or no part in the way they turned out shouldn't prejudice us against their claims to causal significance (though it does of course - because we are incurably vain about such things).

A fresh perspective on such matters is offered by Virtual History, a collection of "counterfactual" essays edited by the historian and journalist Niall Ferguson and accompanied by an introduction which sets out to rehabilitate the "what if" principle of historical analysis. Ferguson rightly points out that counterfactual ideas have a ghostly presence in even the sternest forms of history (you can hardly write about a failure unless you have some notion of what success might have been or how it might have been achieved), but he is not intent on an exorcism. Instead he wants to restore some credibility to what has been dismissed as a disreputable kind of behaviour. He calls in new scientific theories of uncertainty and chaotic systems to bolster his case but at heart his essay is a plea for the virtues of historical imagination - in particular the human capacity to see that things might have been otherwise. Only by doing this, he suggests, can we restore the historical past in all its flux and uncertainty.

Ferguson is rather severe himself about the more trivial applications of such ideas - what's known as the Cleopatra's Nose theory of history, whereby an ugly Queen would have altered the destiny of an Empire. But one of the things that is true of counterfactuals is that they tend to push the historian away from large-scale ideas of causation to a more microscopic vision. This is because counterfactuals must have a point of departure, a moment where the actual world ends and the historian's alternative begins, and that moment is likely to be relatively circumscribed. (Though a "moment" remains a very flexible measure of time - in the 18th century it could be used as a synonym for a second but that seems to reflect a massive temporal devaluation: in 1398 you can find an explanation that "an houre conteyneth foure poyntes and a poynt ten moments", in other words, one and a half minutes).

Under such a speculative light, the topology of recent events looks a little different - and the election itself much less salient because it was never as susceptible to plausible alteration. Even before the result, it was far less likely that Labour would lose than that they would win (a fact reflected by the purest recorders of historical expectation - the bookmakers). Perhaps, then, the real historic moment, the point at which the dice were cast, lies much further back - on Black Wednesday, as some of the defeated ministers have tried to suggest (putting as much clear water as they can between themselves and disaster) or on the day when John Smith's heart finally gave out, leaving the way clear for our new Prime Ministern