One of this country's problems, even in our supposedly meritocratic age, is the blinkered class prejudice which leads to snobbish sneering at someone's accent, his parents' jobs, his schooling or a host of other factors about which the individual has no choice.

Such is David Cameron's lot, but there is no sense in denying that his background will count against him as a spokesman for the Union. His political allegiance, by contrast, is a matter of choice, and it would be equally foolish to deny that a pro-Union campaign seen as driven primarily by the Conservative Party would be counter-productive.

Some who find it absurd that Scotland cannot produce a respectable political vehicle for those whose views tend towards the centre-right (and suspect that constituency of being larger than either the media or election results suggest) maintain that only after independence will such a political position become tenable. Perhaps they're right. The visceral reaction to the word "Tory" north of the Border seems as automatic as that of Pavlov's dogs to the dinner bell, except that Scots whine and growl rather than salivating.

If it's true that conservatism in Scotland can't recover until after separation from England, and given that Mr Cameron would have a permanent Tory majority south of the Border if that happened, there doesn't seem to be much of a case for the Union even for the party which used to have the word Unionist in its name. Indeed, some moderately conservative Scots and some vehemently conservative English nationalists take just this line.

So it is rather principled of Mr Cameron to ignore his own party's interests on both sides of the Border to argue for Scotland's role as part of the United Kingdom. The Labour Party, by contrast, is in an entirely different position. The end of the Union would be an unmitigated catastrophe for them, and yet their leadership – Johann Lamont and Ed Miliband alike – is not only as clueless and incompetent on the issue as it seems to be on all other issues, but has scarcely noticed the implications.

Nor has some other prominent figure – Lord Reid and Lord Robertson are the names most often bandied about – from the days when Labour had some semblance of a grip on current affairs stepped forward to make the case against the Nationalists' programme. It was revealing that yesterday's editorial in the Sunday Herald was headlined: "Anti-independence team in disarray."

It was also profoundly insightful. Politicians are much given to talking – as Mr Cameron did yesterday – about "a positive case for the Union" and commentators (I'm one of them) have been asking for the same thing for some time. One would think that since a majority of voters is still not convinced of the merits of independence – that's not just my view, or the opinion polls', but the First Minister's, because otherwise he would call the referendum at once – that there would be no shortage of arguments.

Instead, the SNP have brilliantly changed the terms of the debate so that the burden of proof is on the Unionist – sorry, anti-independence – side. Anyone asking obvious and necessary questions about Scotland's future defence arrangements, position within the EU, currency, the share of the UK's debt we might inherit, the boundaries of North Sea oil and gas fields, public pension entitlements, immigration policy or the thousands of other issues which will need to be untangled, is challenged to present a "positive case".

Since nobody has yet been willing to do this, I'll try to make a modest start. The first positive point in favour of the Union is that, though we may not all like the status quo, we know it. This is not a negligible consideration, nor is it timidity. To bear in mind the history of the benefits we have gained by being part of the Union is not to be backward-looking or say we should "always keep a-hold of Nurse/ For fear of finding something worse".

It is a strategic mistake for Unionists to bang on about whether Scotland is subsidised by England. It is, a bit, compared with many English regions (though London is subsidised more), but then we have Glasgow to contend with, as well as huge remote areas such as the Highlands and Islands, which demand higher spending. The benefit of the United Kingdom is that such costs can be shared among a much larger population; the Union gives freedom of movement, lack of tariff barriers and equal benefit, healthcare and pension entitlement to all citizens.

This should be stressed as a positive advantage, not as a claim that the Scots couldn't afford to go it alone, or that they are subsidy junkies. It's the kind of model the SNP produced as their trump card when they advocated independence in Europe in the late 1980s, but it had already been running for almost three centuries with our immediate neighbours.

And it is not scaremongering to point out that those arrangements with the EU, and for that matter the G8, Nato and other international bodies, are a boon that we already have. An independent Scotland might very well manage as well as Norway, but it might not: I doubt we'd end up like Greece, but its example at least shows what can happen in a currency union when public spending and borrowing are unrestrained.

In the end, the economic arguments about Scottish exports to England (the vast bulk of our trade), the number of English people settled here or the number of Scots down south are secondary. What matter most are the personal ties which the dry statistics can never represent, and the accumulation of sentiment, history and tradition which that bond has created.

That positive case for the Union is not one which any convinced Scottish Nationalist will agree with, but it is the most forceful of them all: the positive case for the Union is that most Scots do not want to abandon it. If Mr Salmond doesn't like or believe that reason for the Union, he can test it at once with a straight yes/no referendum question – and, as things stand, he'll still get to complain that the answer isn't positive enough.