Once upon a time the idea of a "Labour left alternative" to Gordon Brown would have sounded like a joke. Back in the days when he was publishing little red books, and long before he discovered the property-owning democracy, the Prime Minister-in-waiting was everyone's favourite alternate, the messenger from a possible future.

Those were days of hope, some of the time, not least because hope was the only clutchable straw on offer. Labour did not advertise its residual socialist tendencies, but it did not apologise for them. In that half-forgotten era, Gordon Brown was the young, brilliant man who wrote polemics on greed - imagine this, in 2007 - as a moral issue. He also behaved as though he meant every word.

Does the same man stand before us now? Many in the Labour Party are desperate to believe it. They think they have paid their dues, and squandered their share of ethical capital, in a brutally pragmatic decade. They believe they have swallowed enough, and more than enough, for the sake of power.

You can almost visualise the scales being brought into balance in the minds of the forever-loyal. Three election victories against one lousy war. A consoling devotion to the NHS weighed against demented public-private partnerships. An evolving belief in devolution almost drowned out by a one-man Blairite band. Constitutional reform versus a cash-and-carry trade in elite baubles. And the rest.

Brown, whether he knows it or not, is expected to offer a clarity of purpose now. Under his leadership, Labour, or so it is believed by the devout, can stop pretending. After all, the unbeatable anointed offers one shining virtue. Anyone but Blair? So who else but Gordon?

This must be comforting, if such is your need. Blairism and the Labour Party resembled the most dismal sort of arranged, sometimes-convenient, marriage. Neither was happy or contented for long. Whether a fourth British election can be won remains to be seen, but Brown represents one of the most cherished words in the lexicon of traditional Labour. He is, with all that this implies, "authentic".

Or so they tell you. What they cannot tell you is how they can possibly know. Over the past decade Brown has managed the strange trick of being a highly visible, interventionist Chancellor while remaining entirely inscrutable. He has performed the feat with some skill, but he has also posed a riddle for his supporters. If they cannot tell us who this man truly is, how can they tell us what Brown's Britain might be like?

Earlier this year I reviewed a book of the Chancellor's speeches. It was a volume designed to establish, or re-establish, his credentials as a statesman. It contained many admirable sentiments on issues such as Third World poverty. It had very little to say - big surprise - about an old friend named Blair. Yet even after several hundred pages, Brown himself seemed like the merest sketch of a personality. Even by the minimal standards of modern political rhetoric the man himself appeared barely to exist, as though he regarded his own identity as a kind of irrelevance.

After Blair, that counted, still counts, almost as a relief. Better a Prime Minister with a passion for ideas than one desperate to sound merely passionate. These days Brown's best bet, clearly, is to provide a stark contrast with a discredited predecessor. The trouble is, Scotland and Britain both need to know who this leader is. We already know the person he is not: hence yesterday's "bounce" in an opinion poll.

John Major got one of those, you may recall. He was not, palpably, Margaret Thatcher and for many, for a while, that was enough to be going on with. But the Major years ended in sleaze and shambles. His personality remained opaque (I'm being kind) while the man himself became a Spitting Image joke. Brown needs to learn the lesson, and quickly. Policies may matter more than personalities, but an apathetic age is unkind to impersonal leaders.

So what do we know? There will be less spin in future, or so an inveterate spinner promises. Power will be distributed from the executive to the legislature, says an obsessive centraliser. Parliament, party and people will matter more, at least according to one who believes he knows what's best for each of these estates. And Britain will cease, finally, to be a disputed theoretical construct. That's the big idea.

Well, he would say that, wouldn't he? He would say that, in particular, if his opponents have decided to make an issue of his Scottishness, and if Scotland's voting habits have become unreliable. That book of Brown's speeches contained a great many words about the value and importance of Britain. You could dismiss them (I did) as self-serving. After all, too many people have wrapped themselves, too often, in the Union flag. But how's this for a question: what if the Chancellor means what he says?

What if this country never again goes to war without the explicit approval of all our elected representatives? What if power is devolved coherently, to the NHS and beyond? What if Brown gets us out of Iraq? And what if the Nationalists of these islands are about to encounter a Unionist who actually believes in the case he makes? The experience of Scotland's elections suggests certain rhetorical questions can be overdone, somewhat. Brown treated the arguments for self-determination, never mind the electorate, with a kind of inept arrogance. Sophisticated he was not.

Yet nor was he master, entirely, of his own destiny. Blair was still taking his curtain calls and Jack McConnell was too busy striking columnists from his Christmas card list to grasp the scale of the problem. One debacle, even a debacle without recent precedent, does not count as a judgment on Brown. Not yet.

The socialism of his youth will not reappear the moment Blair departs. His ability to trust colleagues will not reassert itself, least of all where the Treasury is concerned, when he becomes the undisputed first among equals. He will not give Alex Salmond the time of day, if he can help it. He will struggle to achieve a rapport with voters jaded beyond words.

There remains, nevertheless, something tantalising about Brown. He gives a sense of a man bursting with pent-up energies. He behaves as though he has done all his homework (with optional revision) and can't wait to sit the test. He will melt the chocolate soldier the Tories have chosen. But then he will have to convince everyone, on each side of all the borders, that this Britain, idea and reality, still has meaning.

I don't believe he will succeed: the front page need not be held. At a very rough guess, nevertheless, I give the man this much credit. He has attempted to mesh ideas of common justice with an idea of a common, multicultural national home. He has attempted to hint that the Blair years, for which he was in large part responsible, were an interregnum. He is also doing the people the courtesy of offering an argument.

I'll do him the equivalent courtesy, in a small way, by disputing even the need for Brown's Britain. Forget that, for now. Iraq remains the test, but imagine the novelty: politics for grown-ups. Whatever next?