Early in 2007, there were no forecasts from respected think tanks to suggest that the British economy was about to fall into abject ruin.

Just a couple of those warnings might have been handy. As it turned out, even the government of the day had not the faintest idea about the relationship between economic reality and the financial sector's fantasies.

In 2010, when George Osborne boasted that the independent Office of Budget Responsibility would keep all politicians on the straight and narrow, few predicted - a word now rich in irony - that the OBR would establish an unmatched record for being dead wrong, time after time, in its forecasts.

It now does a fine line in retrospective rationalisation, much like the tradesman who tells you that he's been given the wrong parts, but you wouldn't bet the house on the fiscal prospects outlined by these experts. Forecasting is an art, not a science.

A think tank has to have something to think about, however. Equally, a political party claiming the right to shape a country's future has to sketch a plausible picture of what the party, and the party alone, can achieve.

Thus we have the Institute for Fiscal Studies (IFS) extrapolating from disputable datasets and some questionable assumptions to show that an independent Scotland will have to cut spending hard or raise taxes. On the other side of the fence stand Alex Salmond and his Finance Secretary, John Swinney, asserting that a Yes vote next September will liberate the economy. Both protagonists claim to have seen the future. Someone is in error.

It is entirely possible, in fact, that both the IFS and the SNP have misunderstood the difference between what is possible and what is likely. Taking a set of assumptions involving tax revenues and demographics, the IFS discerns only shades of gloom; even its "optimistic" scenario leaves Scotland with a hefty deficit. Messrs Salmond and Swinney meanwhile cannot help but look on the bright side. For them, an inherently wealthy country needs only freedom of economic choice. The idea that Scotland might be essentially neither better nor worse off in the aftermath of independence, muddling along much as usual, is too dull to receive much attention.

What can be said is that forecasts of tax revenue are often wide of the mark, that predictions of population changes and their effects have rarely served Scotland well, and that the data used by the IFS are not above reproach. The Scottish Government Expenditure and Revenue figures and the Treasury's Public Expenditure Statistical Analyses, on which the IFS depends, have each had their share of criticism for being, at best, incomplete.

That doesn't invalidate the IFS case. It doesn't answer the claim that independence could leave Scotland attempting to bridge a fiscal gulf of £3 billion at best and £10 billion at worst. But it remains a caveat worth bearing in mind. "If A then B then C then D" can make for strange logic.

The SNP case will be dismissed by the party's opponents as heroically optimistic (or words to that effect). It might cause even supporters of independence to wonder about the "left of centre" tag bestowed on Mr Salmond. In this guise he and Mr Swinney certainly treat taxation in terms that would not cause many philosophical difficulties for the average Conservative. Mr Osborne is as keen on cutting corporation tax as any Nationalist politician.

Here, nevertheless, are those famous economic levers in an SNP report devoted to that topic. Here, too, is an argument founded on Mr Swinney's belief that in the three decades between 1977 and 2007 Scotland's economic growth lagged behind that of other small nations (of the independent sort) by 3.8% of GDP or the more eye-catching "£900 per person". We are "doing okay", as the Finance Secretary puts it, but could be doing very much better. Yet the IFS oracle fails to recognise such statements.

The argument is, in any sense of the word, fundamental. The institute believes that even granted a lot of luck Scotland would struggle to remain truly viable. The First Minister and his Finance Secretary say the opposite: the Union is the worst of all possible worlds. Unshackle economic decision making and Scotland could, at the least, be a match for other nations of equivalent size.

Set a general belief in independence aside and the SNP case still makes a certain intuitive sense. The Scottish problems identified by the IFS are hardly unique; Scotland's advantages seem enviable to most small countries. Where is the problem in aspiring to at least be on level terms, economically, with some of our north European peers? What do they possess that we somehow lack? More precisely, what is it they do that we, lodged within the UK, have failed to do?

The SNP answer involves taxation, an industrial strategy, regulation and inward investment. Stress is put on manufacturing and a more efficient - not hard, you might have thought - tax system. The broad idea is to tailor policy to suit Scotland's specific needs. Mr Salmond aims, as it were, to secure a bespoke economy. The UK alternative, he says, involves still more cuts to public spending. That claim, at least, is probably beyond dispute.

The belief that London has neither the interest or ability to enact specific measures to meet specific Scottish needs is hardly new, but rarely rebutted.

Some might say the problem could be answered by still more devolution, though whether that kind of dispersal of power would differ much from de factor independence is a question few are prepared to answer. Oddly enough, however, the case presented by Mr Salmond and Mr Swinney has as much to do with basic political attitudes as it has to do with constitutional arguments.

What the SNP offers is an interventionist state in a style not seen in these islands for decades. The chance to cut corporation tax has more to do with a government's role in the economy than with a philosophical belief in low taxation. Even a phrase such as "industrial strategy" has not been used in earnest for years by UK governments. The SNP document talks of "transformational" policies, of priorities for a re-balanced economy. This kind of talk is old but, in 21st century Britain, refreshingly novel.

It finds no echo whatever in the IFS analysis. The think tank and the SNP might be talking about different worlds, far less different visions for one country. The IFS, typically, takes no notice of that strange political phenomenon, "events", change, the unexpected. The Nationalists falter in their optimism only when conceding that a new Scotland will not be built overnight.

Economics is not a science, despite claims to the contrary. It proceeds from assumptions and beliefs that reality is liable to contest. You might say the same about Nationalism. But add: it believes above all in itself.