Anyone who has reached an opinion about independence, for or against, has dealt with one argument above all others.

You could call it the Too Poor Hypothesis. It goes something like this.

If three centuries of Union have left Scotland so enfeebled that it cannot dare risk recovering its statehood, what's so great about a shareholding in UK plc? If, on the other hand, those centuries of Union have been such a boon, why is there any "economic debate" over independence? We can't be prospering and too poor simultaneously.

Recognised or not, it's a problem for both sides. Unionists say either Scotland is an economic basket case dependent on Westminster, one that would collapse without the UK's support, or Scots have done terrifically well from the deal and would put all at risk by a wilful act of self-determination.

Nationalists, SNP Nationalists at any rate, meanwhile tell you Scotland is a rich country, one of the richest in the world. They want voters to understand there is nothing to fear from independence, things – as someone once said – can only get better. The risk is people will wonder why they should give up a good thing. If Scotland is rich, the Union must be working. All of this leaves the rows over the economy, present and future, looking a little odd. If the "positive case" for the Union holds up, economic factors are a side-issue. After three centuries of blessings, we can easily stand on our own feet. Correct?

If optimistic nationalism has it right, meanwhile, the economic facts should be self-evident and reassurance unnecessary. People living in a rich country shouldn't need to be told there is nothing to fear.

At the heart of everything is the historical question of Union, good or bad. Neither side seems able to come up with consistent answers. The SNP can't quite say the deal has been a catastrophe from which Scotland has somehow managed to emerge as a rich country. Unionists can't claim a miserably dependent backwater has enjoyed three centuries of glorious success. The upshot is a certain, shall we say, intellectual promiscuity.

Yesterday, the SNP fired off another salvo in what we should probably call Document Wars, the now regular exchange of analyses and arguments over self-determination. This one, "Scotland's Economy: the Case for Independence", is dedicated to a pair of propositions that are not held to be contradictory. First, the country has considerable economic strength; secondly, the economy is being hobbled by an inept Westminster.

In his foreword, Alex Salmond does not duck what he calls the "paradox". He writes: "Despite our significant array of human, financial and natural resources we are not as prosperous a country as we should be." The First Minister notes our long-term growth rate has lagged behind the UK; economic performance has trailed behind that of other countries of a similar size; and the gap between rich and poor has been widening.

Mr Salmond quotes his own Fiscal Commission Working Group of economists (not for the first time): "By international standards Scotland is a wealthy and productive country. There is no doubt Scotland has the potential to be a successful independent nation." That's good to know. What it fails to explain is how the country came to be "wealthy and productive" if the Union has been a disaster. Or is the SNP saying, in effect, that economic and political union with England, Wales and Northern Ireland has been successful, just not successful enough? As a purely economic case, that one sits on the disputable borders of history, where things grow misty.

Fog certainly seems to have settled over the No camp. The SNP's document makes six central points. It argues that Westminster's decision to cut capital spending – taken by the Coalition and Labour before it – was rank stupidity in the face of a downturn. Events and changes of heart in London have proved Mr Salmond right in that regard.

The paper adds that the failure by London governments to establish an oil fund was one of the bigger historical mistakes. The point is difficult to contest. There is no Scottish or British oil fund. Norway's problem, in contrast, lies in trying to work out how many hundreds of billions, possibly trillions, it will have to spare when oil runs out.

The paper also assails credit booms, debt orgies, rising income inequalities, the absurd imbalances that place London at the heart of economic activity, and the mad obsession with austerity at all costs. There should be plenty in that lot for a Unionist to address.

Instead, Alistair Darling, leading the Better Together campaign, complains the paper says too little about the future, only that "it's all Westminster's fault". Why the second of these tactics should be illegitimate is mysterious. Has Mr Darling decided the present Government is not at fault, directly, continuously, for the state of the British economy?

Westminster failed to establish the oil fund. Westminster – perhaps even a Labour Chancellor – cut capital spending, let banks, credit and inequality run wild, and adheres now to austerity despite a mountain of contrary evidence. None of this was made in Scotland. Westminster is blamed because – what's the simple way to put this? – Westminster deserves blame.

Mr Darling is on firmer ground with his complaint that the SNP has not laid out precise plans for the future, plans he can kick around. Perhaps Labour should put up a few plans of its own, then, so those determined to vote No can see what their decision is likely to achieve. Things have gone quiet on that front. Unionists continue to suggest devolution will be "improved" if independence is rejected, but fail to say how or when.

None of this, the SNP document or the predictable attacks from Better Together, is exactly to the point. All we know, despite the rhetoric, is that one side says Scotland would be better off without the Union and the other says things would be worse. Most of us had already grasped that part of the argument.

The First Minister is undoubtedly right when he insists Scotland is in better shape than some Unionists would have you believe. Mr Darling has a point – though he could articulate it more clearly – if he believes some Scottish problems are Scottish made. But that would lead us to the interesting topic of Labour dominance over Scottish affairs for decade after decade. I doubt the former Chancellor's party is in the mood for that particular conversation.

Speaking in Falkirk yesterday, Mr Salmond all but said Scotland has nothing to fear but fear itself. I happen to agree with that, broadly speaking. But since Mr Darling has mentioned the future, I would add something else to these ritualistic exchanges. What troubles me most is not the future of Scotland, but the future of a UK with Scotland still attached.

Now there's a real basket case heading pell mell towards another financial sector calamity. Better Off Out Of It feels like a slogan whose time has come. The demand would include the pound, if anyone's asking.