Sometimes it's worth stating the obvious.

Whoever decided that it was a good idea to have David Cameron outline his pitiless attitude towards welfare just as Alistair Darling was advertising the benefits of Union had not exactly thought things through.

The contrast was stark, the message unmistakeable. If the Tory Prime Minister has his way the Britain lauded by the former Labour Chancellor will shortly cease to exist, referendum or no referendum. Benefits and social security – the welfare state – are top of the list of reserved powers in schedule 5 of the Scotland Act 1998. No amount of tinkering with devolution will alter the fact.

Mr Cameron's Conservatives will not necessarily win the next General Election, of course. The speech was mostly speculative, and probably designed to persuade the Prime Minister's back-benchers that he remains part of the tribe. As a statement of intent it could turn out to be as worthless as the promise of a referendum on Europe.

As a statement of attitudes and values, however, Mr Cameron's assault on life's victims was unambiguous. Did he mention that (as of 2010) tax evasion was costing the Treasury £15.2 billion while benefit fraud was £1.1 billion? Did he explain that 87% of the housing benefit he hopes to claw back from those under 25 goes to people who are in work but hellishly underpaid? Did he report that, according to HMRC, corporate tax avoidance is worth at least – an important qualification – £5 billion a year?

Of course not. Having reached the limits of his righteous indignation by denouncing the petty infractions of Jimmy Carr, Mr Cameron had no interest in that kind of context. Nor, it seems, did he pause to wonder over the effects of his speech on the campaign to keep Scotland within the United Kingdom. Instead, he was using recession – the disease his Government promised to cure – as cover for an assault on collective responsibility.

This was a speech from the shrunken heart. It was a demonstration of Mr Cameron's beliefs and desires, a description of the Britain he would like to create. Crudely, it was a summation of what you get if you vote Tory. Anyone liable to be moved by Mr Darling's "positive case that celebrates not just what makes us distinctive but also celebrates what we share" should bear a detail in mind: if you believe the polls, the Prime Minister's workhouse welfare is popular south of the Border.

It might be popular here too, of course: the Westminster Government has a masterly way with populism and double standards. More important is the fact that Scottish voters have no real say in the matter, for or against. We continue to return a single, mainly-decorative Tory MP. We did not, by any calculation, elect the Coalition Government, or lend much support to its constituent parts. Yet again, those issues were out of our hands.

If that sounds like an argument left over from the 1980s, so be it. Nothing in Mr Darling's "positive case" alters the arithmetic, or the underlying attitudes that create the electoral numbers. Devolution has produced nothing more than a contradiction in terms: limited self-determination. Schedule 5 leaves a London government, a Cameron government, unfettered in most of the things that matter, from warfare to the economy, welfare to employment.

How does that square with any attempt to be "positive about our links with the rest of the United Kingdom, through families and friendships, through trade and through shared political, economical and cultural institutions"? Mr Darling would be the first to argue that a coalition bereft of real Scottish support has been bad news for each and every one of those institutions. Still he invites us to embrace a relationship that renders dire consequences inevitable.

The former Chancellor is an honest man. I believe him when he speaks of his sense of shared identity, just as I believed him when he infuriated Gordon Brown by telling the truth about the economic crisis. He has seen the British state from the inside, and finds value in it. He has that prototypical Labour belief in the common interests of all the peoples of these islands. I hear what he says, but I also hear what Mr Cameron has to say. It comes down to this: independence might well be risky for Scotland, but not half as risky as a future within the UK.

That hardly qualifies as a positive case, of course. Nevertheless, the devil you know loses his appeal, somewhat, when he turns out to be as devilish as the current Prime Minister and the political tradition he represents. The self-evident fact remains that a No vote in 2014 would be a vote for the better-than-evens. Mr Darling was right to argue that the SNP has been hazy, if not confused, about certain policies. He neglects to observe – because it doesn't suit the No campaign – that no-one will be voting for Alex Salmond's party in 2014. Whatever the wording, that will not be part of the proposition before us. To pretend otherwise is akin to asking about the policies Scottish Labour would pursue in the event of independence.

Speaking in Edinburgh, Mr Darling said: "When Scotland votes in the referendum, we will face a historic choice which will shape our country and our families' futures, not just for the lifespan of a parliament, but for generations to come". We can agree on that. The decision could not be more important. It will be no less important when Mr Cameron and George Osborne set about carving another £10 billion from the welfare budget in this sceptred tax haven.

Better Together, says the slogan. We've heard worse. The line about "a one-way ticket to send our children to a deeply uncertain destination" also counts, no doubt, as a novel addition to the politics of fear. But we still await an answer to the old, simple question. If the Union has been such a blessing for Scotland, why do Unionists maintain that we are still too weak and witless to stand on our own two feet? After three centuries, an answer is overdue.