ON a visit to Edinburgh in February 2012, the Prime Minister declared himself to be "a Unionist, head, heart and soul.

I believe that England, Scotland, Wales, Northern Ireland, we are stronger together than we ever would be apart."

He then turned to the matter of national identity. "The ties of blood," he argued, were "actually growing thicker." Unionists have long been adept at making practical - chiefly economic - arguments for the preservation of the UK, but in this speech, David Cameron was attempting to reclaim some of the cultural or spiritual dimension that has generally been the preserve of Nationalists. This, he said, was "a question of the heart as well as the head".

Pro-independence supporters, by contrast, have worked hard to shift the referendum discourse away from issues like national identity and towards political "values", stressing the why - the policy objectives - of independence rather than the how, the process). The late Sir Neil MacCormick, a prominent SNP politician, drew a distinction between "existential" and "utilitarian" Nationalists: the former desiring independence for its own sake, the latter in order to deliver a better society.

According to Deputy First Minister Nicola Sturgeon, however, most of today's SNP members are "an amalgam of these two strands". In a thoughtful speech at Strathclyde University towards the end of last year, she argued that while she had "never doubted that Scotland is a nation", for her "the fact of nationhood or Scottish identity is not the motive force for independence. Nor do I believe that independence, however desirable, is essential for the preservation of our distinctive Scottish identity."

Rather, Sturgeon's belief in independence stemmed from principles of "democracy and social justice". People often ask, she added, why someone like her - anti-Trident, pro-social justice and from a west of Scotland working-class family - had joined the SNP rather than the Labour Party. The reason, she replied, was: "You cannot guarantee social justice unless you are in control of the delivery." For her, independence was more than an end in itself, since only by being independent could Scots "build the better nation we all want". Looking ahead to the referendum, she appealed to Scots to base their decision "not on how Scottish or British you feel, but on what kind of country you want Scotland to be and how best you think that can be achieved".

It is an interesting contrast: a Unionist prime minister making an emotional, almost Nationalist pitch on behalf of the UK, and a Nationalist deputy first minister eschewing sentiment and making a hard-headed, practical case for Scottish independence. Not all Nationalists, of course, agree with Sturgeon's analysis. Asked to articulate the essence of his political philosophy, the First Minister replied: "I think the case for independence is a fundamental one. It is about Scotland as a nation and nations have a right to self-determination."

Asked if he believed independence was of value even if Scots gained no economic advantage from it, he replied: "I fully agree. A sense of identity, a new confidence in a proud nation with a strong sense of social justice, a good global citizen: these are all attributes which Scotland aspires to through independence. And, of course, the fact that we will flourish economically is also a welcome bonus!"

So while Salmond leaned more towards existential Nationalism, his deputy claimed to be more of a utilitarian. A 2012 survey of the SNP's membership, however, appeared to put most members in the former camp. When asked if "all else should be secondary" to the "primary goal

of independence", 71% agreed or strongly agreed. But, of course, Sturgeon's pitch, and that of the party generally, looks beyond the converted.

The Nationalist impulse, and to an extent also that of the Unionist, is perhaps better understood as an article of faith, and faith - like the religious variety - owes more to instinct than utilitarian analysis. Unionists are a bit like lapsed Catholics, strongly committed to their faith despite many obvious failings, while Nationalists resemble Wee Frees, determined to plough their own furrow regardless of public indifference and theological inconsistencies. Scottish Tory leader Ruth Davidson said even Conservatives share "the Nationalists' faith in Scotland's future; but our faith is not blind to the facts".

The religious analogy is not new. As Michael Kelly argued, independence to Nationalists is an act of faith: "They have no need to explain it or defend it among themselves. But they are outraged when those not of their religious persuasion challenge the very basis of their belief."

Similarly, the former Canadian Prime Minister Jean Chretien recalled that ahead of the 1980 Quebec referendum it became "almost a religion for them [Quebeckers] to have their country, so on the No side you're breaking the dream of someone". Referendum expert Dr Matt Qvortrup used the Canadian province to illustrate his belief that votes were "won by emotions, not by economics".

In the 1980 referendum, the Parti Québécois had pushed economic arguments and lost by 20%, while in 1995 the charismatic Lucien Bouchard had stressed the cultural differences between Quebec and the rest of Canada and come within a whisker of victory.

One Downing Street adviser appeared to concede this point: "What the Canadians tell us is they handled badly the emotional angle, and that's what caused the dramatic narrowing late on in the day." British Unionists, it seems, are determined not to repeat the same mistake.

One article of Nationalist faith has it that England and Scotland, or rather Scotland and the rest of the UK, are "distinctly different" (according to the SNP MSP Fiona Hyslop), electorally, politically and culturally, something implied by Alex Salmond's complaint in the summer of 2011 that the BBC kept referring to "UK riots" when in fact they were specifically "English" (there existed, he argued, "a different society in Scotland").

In electoral terms, of course, Scotland and England have indisputably grown apart during the last half-century. Political scientists first noticed a divergence in the 1960s when the "Scottish Unionist" (or Conservative) vote began to decline and the SNP vote started to rise. The turning point came in the October 1974 general election when the SNP got 30% of the vote and 11 MPs. Although the Nationalist vote subsequently declined, Scotland, for much of the next few decades, had a four-party system of politics.

The 1970 general election had also given rise to the "no mandate" argument. Until then, UK governments generally enjoyed a majority of seats in both Scotland and England, but Edward Heath's Conservatives had just 23 Scottish MPs, prompting grumblings that his administration's writ did not extend north of the Border.

This was curious, for when the reverse had been true - for example, in 1964 - the English did not complain about an arguably greater electoral injustice. As the political scientist Richard Rose put it: "The Tories did not issue demands for the creation of a devolved English assembly to meet in Winchester, on the grounds that Labour was unrepresentative of England. Losers as well as winners accepted that the power of government belonged to the party winning the most seats in Britain overall."

That consensus, however, broke down from 1979 onwards and reached its peak in 1987 when Margaret Thatcher secured a third term with just 10 MPs in Scotland (and 25% of the vote). Ironically, the point was pushed by Labour opponents of the Iron Lady as well as Nationalists, but it undoubtedly gained resonance: Thatcher was perceived, whatever her constitutional legitimacy, to lack "consent" to govern.

Devolution was supposed to settle this "democratic deficit", for if Scots were able to control education, health and so on, then they could opt out of the Thatcherite agenda (even if in other important respects, such as the economy, this would not be the case). Only after 2010, with the emergence of a UK coalition government, did the "no mandate" argument resurface. Even factoring in the Liberal Democrats, the new administration had just 12 MPs in Scotland (including only one Conservative) and around 35% of the popular vote.

As Nicola Sturgeon put it in 2012: "I simply do not believe that Scotland should have to put up with long periods of UK government led by a party we did not vote for. It is surely democratically indefensible that although the Tories have never won a majority of votes or seats in Scotland in my entire lifetime - or even come anywhere close - they have governed Scotland for more than half of my lifetime."

Thus it is a central argument of the pro-independence case that having opted out of the Westminster system, Scotland would always get the government it voted for.

Sturgeon et al frequently point to surveys showing the Scottish Government to be "trusted" by more Scots than the Westminster equivalent (71% compared with 18% according to the 2011 Scottish Social Attitudes Survey). This is undoubtedly true, although Scottish turnouts in UK elections since devolution have actually been consistently higher than those for the Scottish Parliament (63.8% in 2010 compared with 50% in 2011), which implies Scots still consider Westminster elections of greater importance.

The rise of UKIP in England (and to a lesser extent Wales) has also provided the SNP with a convenient narrative. UKIP have virtually no presence north of the Border so when, last May, the party secured a quarter of the vote in the English council elections, the SNP's Kevin Pringle tweeted that Scottish politics "looks very different from Westminster politics this morning".

And when, in the wake of this election, the Conservatives indulged in a predictable bout of infighting over the prospect of an in/out referendum on membership of the European Union, the SNP spoke of the "different directions" being taken by Scotland and Westminster.

Journalist Lesley Riddoch has written of "two sets of values … grinding away at each other" in "almost every aspect of life", something Labour's Douglas Alexander argues is part of a general attempt to convince Scots "that the rest of the UK has become so foreign a place with such different values, a foreign place so lacking in points of deep connectedness, and with so little sense of being neighbours, that we should split apart".

This claim, argues Alexander, "relies on the implicit but spurious assertion not only that we as Scots are committed to social justice, but that our friends, family and colleagues across the rest of the UK are not". It also depends on "the stereotype of voters south of Gretna as Conservative in character, somehow irredeemably different from Scottish voters".

Despite Alexander's critique, his mentor Gordon Brown also laid claim to distinctive Scottish attributes in his 2012 Donald Dewar Memorial Lecture.

He framed his "Scottish values" in terms of the "democratic intellect", an egalitarian belief in making educational opportunity available to all, and the "idea of civil society, of a community where we have mutual obligations to each other and where there is a moral core to the public realm". Brown also argued that those "distinctive Scottish values" had shaped not only Scottish society but the "British Union", shared values in a multinational state.

Similarly, Alistair Darling said he welcomed the independence referendum as a "chance to reaffirm Scottish values and our expression of them in our partnership with our neighbours".

In policy terms there has, of course, been some notable divergence since devolution in 1999. Nicola Sturgeon described it thus: "In the 13 years of devolution, great changes have occurred. We lose sight of them in the pell-mell of politics - but unlike the privatisation process south of the Border, our health service remains true to Nye Bevan's founding principles; our education system has a new curriculum fit for modern teaching and learning; our universities offer education based on the ability to learn, not the ability to pay; and our older people have more security in their later years."

A deficit-cutting Tory-led government since 2010 has naturally provided the SNP with an even more potent contrast.

Salmond said: "We should aspire to be different. In Scotland the poor won't be made to pick up the bill for the rich. When we control our natural assets as a sovereign power, the profit from the land shall go to all. Too many of them have been ill-served by the Union as it currently stands. There is a better way."

That "better way" will be outlined on Tuesday when the Scottish Government publishes its long-awaited (and much-hyped) White Paper on independence.

All the signs are that the same tension - between existential and utilitarian Nationalism - will be present in its 500-plus pages: an attempt to inspire on the one hand, to convince a majority of Scots that independence is worth having in itself, with on the other a blizzard of detail designed to persuade sceptics that a cost-benefit analysis makes leaving the United Kingdom the only sensible option.

Will it work? We'll find out in less than 300 days.