John Major was right when he said that the referendum on Scottish independence in September is more important than next year's general election. Independence is a once and for all decision and almost certainly irreversible. A majority of one for independence, on however small a turnout, will be accepted at Westminster as final.
But what is independence? The meaning is being fudged both by supporters and opponents. The Scottish National party dislike the word "separatism". It argues that it is merely renegotiating a union. In January 2012 the SNP leader, Alex Salmond, said he wanted not only a shared monarchy and shared currency, but also a shared "social union". Independence, then, is not as fearful a step as it might seem, but merely an adjustment to the 1707 Acts of Union.
Unionists emphasise the constraints that an independent Scotland would face. A currency union would almost certainly mean a commitment to banking and perhaps even fiscal union with the rest of the UK. That, in practical terms, would involve London scrutinising the Scottish budget.
Moreover, if an independent Scotland were, as a functioning democracy, able to take its place in the EU - and, whatever the outgoing commission president Jose Manuel Barroso says, it is difficult to see how this could be denied - it would face further constraints. For the member states have agreed to limit their own sovereignty. This means that on such matters as competition, trade, agriculture and fisheries, there is little scope for divergent policies. The idea of independence, unionists argue, is a chimera in the modern world.
But the EU, despite its rhetoric, has not succeeded in establishing a common foreign or security policy. Indeed, in most of the foreign policy crises of the last 25 years - the first Gulf war, Bosnia, Kosovo, the Iraq war - the EU has been divided.
An independent Scotland, therefore, could decide its own foreign and defence policy. The SNP proposes that Scotland should become a non-nuclear state. An independent Scotland could, if it so wished, leave Nato. And we only have to look across the Irish Sea to appreciate that Ireland has considerable scope for independent policies. Whereas in 1914 Ireland, as part of the UK, was a combatant in the first world war, an independent Ireland in 1939 chose neutrality in the second. It makes a great deal of difference, therefore, which country one belongs to.
Independence is not simply a further step in a continuum from devolution, but involves a fundamental discontinuity. Devolution is a matter of degree, independence an absolute.
This has two implications. The first is that Scottish legislators, meeting as representatives of an independent state, would have much more scope for independent policies than Scottish MSPs in a devolved Holyrood.
But, secondly, as a necessary corollary, Scotland would no longer send MPs to Westminster. Scotland would be represented in London not by MPs and by a member of the cabinet, the Scotland secretary, but by a high commissioner. So Scotland would have no political leverage over decisions made at Westminster.
The Irish analogy is again of interest. In 1918, Ireland returned 101 members to the House of Commons, 73 of whom belonged to Sinn Fein. The Sinn Feiners refused to recognise British rule in Ireland and, instead of taking their seats at Westminster, set up their own legislature in Dublin, the Dail. The first major measure affecting Ireland after this exodus was the 1920 Government of Ireland Act, providing for partition and devolved government in both parts of Ireland. The condition of the Catholic minority in the north might well have been improved had the 73 Sinn Fein MPs remained at Westminster to argue their case.
An independent Scotland would have no right to a shared currency or shared social union. Its only right would be to propose them. It would then be up to the rest of the UK, a country in which Scotland would no longer be represented and would have no electoral or political leverage, to decide. The terms of independence could not depend on Scotland alone.
A yes vote would be a vote to disclaim the union. It would not then be possible for Scotland unilaterally to choose which aspects of that union it was able to retain. The nation would have to negotiate for what it now enjoys as a right.
The position of an independent Scotland negotiating with the rest of the UK would resemble that of Norway negotiating with the rest of the EU. Norway is in the position of a lobbyist - sometimes called a "fax democracy", because the proposals of the council of ministers are faxed to Norway for its comments. But whatever these comments are, it is rare for the council to alter its proposals.
An independent Scotland would be a mere lobbyist in Westminster - and would also be in danger of becoming a fax democracy.
Vernon Bogdanor is professor of government at King's College London. His books include The Coalition and the Constitution
Like Norway, Scotland would be marginalised in Europe Photograph: Alamy