NOT so long ago, this column mentioned that it doesn't always do to confuse Scotland with Catalonia.

This week we could probably add that it's unwise to mistake any nation's approach to self-determination with the attitudes embraced anywhere else. Do that and you're asking for trouble.

There are simple beliefs held in common, of course. Self-determination is, famously, a cornerstone of international law. It binds together the United Nations (resolution 1514) and attracts better than its fair share of lip service. No one, if you believe a word, is against self-determination.

But Scotland isn't Catalonia, or the Basque Country, or Flanders. History saw to that. The relationships with the dominant power, political and cultural, formal and informal, are different in each case. A one-size response doesn't fit all. Someone should explain this to Jose Manuel Barroso, president of the European Commission.

In September, pressed by Radio 4's The World at One, on the possibility that an independent Scotland would need to reapply for EU membership, Mr Barroso first declined to speculate. For some reason, the president was reluctant to state the case.

Then he said: "It is a procedure of international law. A state has to be a democracy first of all and that state has to apply to become a member of the European Union and all the other member states have to give their consent. A new state, if it wants to join the European Union, has to apply to become a member of the European Union like any state".

The president had said much the same before, and has said much the same since. The twist is that his words have tended to appear in Catalan, for the benefit of a Catalan audience. Plainly, he has not been prepared to make a distinction between cases. More to the point, he has not been eager to disclose any legal advice obtained by the commission. We'll come to that.

Last week, a leaked letter from the EC to the economic affairs committee of the Lords echoed Mr Barroso, as you might expect. In part, it claimed, "if a territory of a member state ceases to be part of that member state because it has become an independent state then the (EU) treaties would cease to apply to that territory". It counted as a creative interpretation of the 1978 Vienna Convention on Succession of States in respect of Treaties. But that's another story.

This week we heard of a further response from Mr Barroso in a written answer to David Martin, the Labour MEP. The latter had wanted to know if Romano Prodi, the EC's previous president, was right in 2004 when he also said of newly-independent states that "treaties no longer apply to that territory".

Again Mr Barroso affirmed the view: "The legal context has not changed since 2004 as the Lisbon Treaty has not introduced any change in this respect. Therefore, the commission can confirm its position as expressed in 2004". One small question went unanswered: what exactly is "the legal context"?

Strangely, Joaquín Almunia, vice-president of the EC, has been less confident about that. Questioned recently by the Spanish press in a parallel argument, he conceded that he could not give an "emphatic yes or no" as to whether Catalonia would automatically remain in the EU if it achieved independence.

The former leader of Spain's Socialist Party might have been pondering a supplementary question. It is one thing to admit peoples to the EU, but how do you throw citizens out? Where is the legal precedent when a country is entwined with European law, part of existing European territory, and not, in fact, a "new state"?

That's another detail overlooked by those who flourish treaties: Scotland, unlike Catalonia, unlike Flanders, has got one. True, it's old. Granted, it was never much good to begin with. But it provides the constitutional, which is to say legal, basis for the existence of the United Kingdom and hence, if we need to split hairs, for the UK's membership of the EU.

Unionists don't like to talk about this. For bystanders, equally, it can seem like a dusty old argument to bring to 21st century conference tables. But if we are to hear much more about the fine old relationship between Scotland and its neighbour – you can count on it – the origins of the relationship matter. Two sovereign nations, one treaty, two acts – not one – passed between two parliaments to join Scotland in political union with England (and Wales).

Mr Barroso would be less keen to obstruct the Scots, I think, if there was no agitation for self-determination in Catalonia. As things stand, he can say nothing about one nation without risking consequences in the other. He is more worried about attitudes in Madrid than he is about a Coalition Government in London. Equally, when Spain's government threatens to obstruct Scotland's continuing membership of the EU, the real target is Barcelona.

The fact remains that the Catalan path to independence, if followed, would be entirely different from the Scottish path. Properly speaking, Catalonia would be negotiating secession; Scotland would be dissolving a treaty. The EC has not yet got its collective head around the fact, and British Unionists – who can blame them? – are not letting on.

John Swinney, for the SNP, concedes that details concerning future EU membership would have to be worked out. That makes sense, though it would be nice if he could spell out exactly what would be involved. The euro, if it still exists, is liable to be on most lists, but nothing is set in stone. That's part of the problem.

The latest line from Mr Swinney's party meanwhile involves the claim that "following a Yes vote in autumn 2014, Scotland will still be part of the UK. Negotiations will then take place on the transfer of powers from Westminster to the Scottish Parliament along with negotiations on the specific terms of an independent Scotland's continued membership of the European Union". The SNP forget to add: if all goes to plan.

The reality is purely political. The Nationalist claim that Scotland would stand as a successor state to the UK on equal terms with England is rock solid if Britain's constitution matters a damn. So what happens if Madrid, for its own reasons, takes Mr Barroso at his word and insists that "all the other member states have to give their consent"? London's good faith, expressed in the Edinburgh Agreement, would become important. There's a cheering thought.

One fly in this sticky ointment is that Mr Barroso, laying down the law, has yet to publish the commission's legal advice. For some reason he can only do so at the request of an EU member. But London won't ask and Edinburgh seems disinclined to force the issue. Both governments might care to explain, one of these days, why that is so.

Instead, we have the commission's president handing out an opinion, nothing more, to journalists, peers and MEPs. He offers his understanding, such as it is, of EU rules, such as they are, governing a situation that has no precedent. Every state has a right to withdraw from the EU (article 50 of the Lisbon treaty). So why can't one state withdraw from union with another, especially when there is no EU mechanism for expelling a member?

As in so many things, the downtrodden people of England have a right to know.