In our correspondence columns recently a reader from Salem, Massachusetts, one Ed Margerum, appeared to let the SNP have its cake and eat it. Ed's point - which had never occurred to me - was, in so many words, that the fearty factor in voting for Scottish independence would be removed if Scotland made the break with England but remained in the United Kingdom.

At first glance this was not as daft as it sounded. Some of you out there might remember Donald Dewar's fleeting passion for Scottish ''Independence in Britain'' - an idea we don't hear much of these days because Donald was very quickly persuaded to abandon the whole bizarre notion and pretend it never happened.

From Salem Ed argued that as Scotland and England (and Wales) had become the United Kingdom at the Union of the Crowns and not through the Union of Parliaments, an independent Scotland could exist inside the UK because the Queen would remain head of the state and the kingdom would remain united.

Ed's one concession was: ''Great Britain would be history, as they say.''

For a moment I must admit to wondering if Ed was on to something. The SNP would have loved it. The Salem theory appears to reinforce the arguments of Jim Sillars 20 years ago that Europe provides a safety net for a free Scotland. It was Sillars's embrace of Europe and all the constitutional implications raised by the Scotland-in-Europe argument which gave the SNP lift-off in the polls after years of bumping along the bottom.

Today, if Ed was right, the Independence-in-the-UK argument would do the trick again and Scottish politics would provide another great debate.

Labour's protests that the Nats were wreckers bent on ''breaking up the United Kingdom'' would be seen to be ridiculous.

The force of the Salem argument would be unstoppable, especially given that in The Herald's recent System Three opinion poll it was suggested that 40% of prospective SNP voters would prefer Scotland to remain in the UK.

But Ed was wrong. Some quick research and a call to Tom Devine of Strathclyde University, in his capacity as unpaid historical consultant to this column, confirmed that the United Kingdom came into being with the acquisition of Ireland almost two centuries after the Union of the Crowns in 1603 - when the term ''Britain'' became established - and almost one century after the Union of Parliaments 1707.

And yet Ed still did us a service because he unwittingly questioned the nature of independence in today's world. In a loose sense the United Kingdom would indeed still exist after Scottish independence because the two former separate kingdoms would remain united under one sovereign. Similarly ''Britain'' would survive the political separation of Scotland and England, if only as a geographical or cultural description but one which would continue to be used commonly in the way that people still include Ireland in the ''British'' Isles.

Constitutionally or diplomatically these terms might carry no clout but show me where the identity of the United Kingdom is set in stone, far less a constitution. (I am grateful to Joyce McMillan for sharing with us the news that she knows an Irishman so indignant at Ireland being a ''British'' Isle that he refers to us as the North West European Atlantic Archipelago).

True and absolute independence, of course, does not exist. Even the Norwegians, sometimes single-minded to the point of perversity, who voted for independence from Sweden in 1905 and who keep voting against membership of the EU, have to rely on their inter-dependence in groupings like the European Economic Area, the Nordic Council and Efta, not to mention Nato of which they are founder members.

An independent Scotland would be little different in the global clachan.

That is why the Unionist parties in Scotland struggle to sound convincing with their scare stories about a ''separate'' Scottish Army or currency or border posts or foreign missions. They forget that all this machinery of independent nationhood exists already for Scotland, only we don't control it.

We have an Army; all that requires is to integrate it with someone else's, probably the coming EU common force. The single currency is almost on us (we're in the British one already) and borders are coming down, not going up, all over Europe. British foreign missions are Scottish, too, and would almost inevitably be shared with England and Wales or the EU if we became independent. Existing common services such as the BBC or railways would survive; telephones and transport are being Europeanised, and so on. The idea that an independent Scotland would somehow be cast adrift, economically alone, politically neutered and diplomatically friendless, is absurd.

Ed in Salem might be wrong historically but in practical terms he has a point. The United Kingdom and even Britain - whatever this identity which has evolved through history means today - would probably survive an independent Scotland.