There are known unknowns and there are unknown unknowns, as Donald Rumsfeld once said. The dangerous things are not the things that you know you don't know, but the things that you don't know you don't know. We all know, for example, that we don't know how good David Cameron is. In fact, two of my colleagues on The Independent on Sunday have just set off in search of the answer to that question, at book length. But we can already tick two of the most important boxes. Presentation? Good. Positioning? Sound. The bigger question of British politics is: how good is Gordon Brown?
That is something that we don't realise that we don't know because he has been around for so long - long enough for everyone to read into him what kind of prime minister they hope he will be, and even long enough for everyone to realise that he won't be like that after all, but not long enough yet for us to find out what he would be like if he takes over.
In that respect, the Budget failed to live up to its pre-match hype. It has long ceased to do so as an economic occasion. Decisions are taken, announced, postponed or rolled into three-year planning cycles at all times of day, night, month and year. Where once the Budget was a decisive annual moment in setting the economic course of the nation, it is now a largely ceremonial occasion in which minor symbolic taxes are tweaked and old announcements repackaged. Yesterday, though, was supposed to be the preview of the next general election. Brown versus Cameron. Man to man. It was supposed to light up the shape of Politics to Come.
But it didn't. Gordon Brown delivering his Budget speech, and dealing magisterially with the opposition responses to it, is an utterly familiar character. He doesn't engage his opponents and they don't unsettle him. The Cameron onslaught was predictable. Good pre-cooked jokes, but more Judy than Punch, and there is no time for even the best parliamentarians to respond to the detail of the announcements. "Billions raised, billions spent. No idea where the money has gone. With a record like that you should be running for treasurer of the Labour Party."
In any case, Brown can do jokes too. His opening riff about Nicholas Vansittart, the last Chancellor to deliver 10 Budgets, was skilfully done: he was not planning to emulate Vansittart in abolishing income tax - this year.
The Budget, therefore, illuminated neither the answer to the problem of climate change, nor that to the question of how good a prime minister Brown would be. This question is, of course, the obverse of another often-asked question, and the better way to ask it, namely: what is the point of Tony Blair? The point of Blair being mainly that he is a better Prime Minister than Brown would be. This was a point made by the editor of The Economist just 10 months ago when he urged his readers to vote Labour on the basis that Blair would serve a full term. Last week, he secured a few headlines by changing his mind and deciding, despite the electorate's verdict, and his own, then, that Brown was more of "a meddler", that it was time for him to take over.
That is one of the core criticisms of Brown, that he makes simple things complicated and ties up the tax and benefit system in such obfuscation that no one knows whether it is working or not. And yet, every Budget, he remains toweringly impressive when viewed in the long run of history. His claim that this is the first Government to have presided over 10 consecutive years of growth cannot be waved away with jokes, however accidental it sometimes seems.
Yesterday's much-anticipated "clash" between Brown and Cameron - which was more like rival armies passing each other in the night - answered none of the important questions about Brown's governing style, therefore. It was Brown doing well what we knows he does well, and Cameron doing likewise. It told us remarkably little about how they might match up as the leaders of rival prospective governments. The extraordinary fact about Brown's long service near the top of British politics is how rarely he has been tested. He has not been forced to make tough, unpopular decisions. The question of how he would cope with the daily tough choices of the top job is central to the conundrum of the next election. But after yesterday, he remains the great unknown unknown.
The writer is chief political commentator for The Independent on SundayReuse content