Two untruths, one trivial, one large, were both thrown casually out by the Prime Minister in his speech to the TUC on Tuesday. I’ll start with the smaller: a sneaky little fib and hardly worth rebutting, but revealing of character because it was something Gordon Brown certainly knew to be untrue, didn’t need to say, but said anyway just to purchase some minuscule and momentary advantage for himself. In words cut from the text distributed to the media, but which I heard him deliver, he said this: “The current Conservative leader was the principal economic adviser to the Chancellor of Black Wednesday and he stood alongside Norman Lamont . . .”

David Cameron was not, of course, the principal economic adviser to the Chancellor of the Exchequer. He wasn’t an economic adviser at all. He was a special adviser; there were scores of them; still are. They don’t count for diddly-squat, many are barely out of university, their opinions on big economic decisions are neither relied upon nor, in most cases, even sought, and Mr Cameron, as it happens, was 26, an age at which the young Gordon Brown was still railing against Margaret Thatcher’s legacy.

Nice, however, to see him paying homage to her on Thursday.

Anyway, just make a note and file it away: an illustration of how carelessly Mr Brown twists when it doesn’t matter.

Append, though, the question how carelessly he might twist when it does matter, because here’s a bigger if more arcane departure from the truth, ten seconds later in the same speech: “It is because we must never return again to those days when reckless promises that you could simultaneously cut taxes, raise spending and cut borrowing were made . . .”

But you often can cut taxes, raise spending and cut borrowing. Indeed, as Chancellor the present Prime Minister repeatedly boasted that he was doing something like this (though the claim he was holding down taxes was another fib). During times of strong economic growth it is usually possible to spend more, redeem debt and cut taxes. There being more economic activity overall, the tax yield can rise even though tax rates have been reduced. The Exchequer takes a slightly smaller slice of a bigger cake: ergo, more cake for everyone, including him. And a cut in tax rates plus extra public spending may further boost economic activity, creating for a while a virtuous circle. It is the hope of every Chancellor to preside over such a period. In recent decades a number have.

Even I know this. The former Chancellor certainly knows it. He must have been trading on the belief that the trade union brothers didn’t know it, or, if they did, weren’t scrutinising his speech for technical howlers. He made the careless claim not as a serious argument, but as part of a background mood-music he was busy creating: you know the stuff – “Iron Chancellor”, “son of the Manse”, “sorry, lads, no more money” etc. Few were likely to bother to pick that speech apart, but I find Mr Brown’s “whatever gets you through the night” approach to fact and reason disturbing: strangely redolent of his predecessor at Downing Street. Alastair Campbell’s diaries breathe it from cover to cover.

And it leads me to wider pasture. Pace the Prime Minister, it is, as I say, common to be able to cut tax, redeem debt and increase spending. Dangerous, of course, to bank on doing so and irresponsible to promise it; no politician can forecast; and in the present climate it would be unwise to construct any kind of a manifesto upon such hopes. But sooner or later it is likely that the UK economy will be revisited by these benign circumstances.

And when they arrive, what I’d like to know is this: how, in government, would each of our mainstream political parties handle the good fortune? I don’t accept the platitude that only when times are tough do the tough get going. Neither life nor politics is only or mainly about failure and vicissitude. We are revealed also by how we handle success. Indeed, when governing parties of all colours must batten down hatches and weather storms, their habits and plans tend to converge. It is when a team in office have money in the Treasury coffers, credit in press, peace at home and abroad and a comfortable lead in the polls that their true colours may emerge. Now they can do whatever it is they came into politics for. Uncramped by crisis, they can unfold their wings and fly. A philosophy can take shape.

So we are revealed, I think, as much by our daydreams as our nightmares. What are Gordon Brown’s? What might Sir Menzies Campbell’s be? What do Cameron Conservatives dream of being able to do? Ever since an incoming Labour administration in 1997 had to promise not to touch their predecessors’ spending limits, the dreary cry of “Ah, but how would you pay for it?” has stunted political debate in Britain. The moment a party leader raises the prospect of doing something novel or interesting, his rivals punch figures into economic models, print results and shriek: “Gaffe! Black hole!” The Conservative Party seems fearful of breathing a word about anything to which the tag “new Tory spending commitment” might be attached by a gleeful government front bench.

Understandably. Any promise of new spending may seem to raise the spectre of a corresponding cut. But even when times are tight the public finances are far from a neat balance sheet on which a push here requires a pull there. Governments regularly embark on initiatives for which funds have not been earmarked. With a prod and a squeeze, the money is found.

The Opposition are wise to steer clear of manifesto promises. But should this sterilise all talk of their hopes? What law of political campaigning forbids opposition leaders, or indeed prime ministers, from sharing their what-ifs, their more optimistic scenarios, their dreams? How about a Green Paper manifesto in the appendix to the real thing? Crossrail for London? A 200mph spinal railway to Scotland? A road-tunnel from Stranraer to Belfast? A multibillion-pound blitz on green self-sufficiency in energy? Free buses in congested cities? A huge salary bribe to draw the cream of the teaching profession into struggling inner-city schools? A new Royal Yacht? A no-expense-spared programme to crack open and rescue the underclass? A real leg-up for the cash-starved postgraduate sector in our universities? A new national park? Four more national forests? A raising of the tax threshold to £10,000? A golden age of prison education?

Each leader will have his own ideas, and though some aims will be common to all, when it comes to priorities you may find the geographies of their dreamlands recognisably different. This matters. It tells us where they will head when, uncaged by accountancy, they can. Nobody’s going anywhere much today, tomorrow, or perhaps at the next election. But after that, who knows? I want to.

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