It's not necessarily the joining itself, as rejection looks highly unlikely, but the rules of the game once you are invited to play.
When you sign up to join an organisation you agree to the rules of membership.
Whether it's the bingo in Partick or a tennis club in Clarkston, rules are rules, and the rules are the same for every member.
For the EU, the rules include agreeing to join the euro and not charging other EU citizens for university education if you offer it free to your own.
Except the EU is not like other organisations and there are different rules for different members.
Not every country is in the euro - some who are might even wish they weren't and there are opt-outs which make it far from a strict, uniform organisation.
So it could be possible for Scotland, like the UK, not to be in the euro and it could also be possible to negotiate a deal where it is allowed to charge English students. Possible, yes. But the question is, is it right and is it likely?
Is it right that one of the first acts of a new independent Scotland would be to discriminate against young people from its nearest neighbour and closest ally.
THE reasoning is understandable - which is to protect free university places for Scots when the neighbour charges among the highest fees in Europe.
But is it fair? What about Irish students, who are charged at home. Would they be charged too? The same justification could apply as it would for charging English students as they are geographically close and English speaking.
What about Poland? Plenty of young Poles come here for economic reasons. It costs £2500 a year to study in Poland and £10,000 for a medical degree, so free education in a country like Scotland, where there are plenty of fellow nationals, would be attractive.
The problem is while it is technically possible in an organisation like the EU to negotiate special circumstances, is it achievable?
And in politics it's all about clout, so what is achievable by a big member state such as France or the UK, might not be achievable by a smaller state such as Portugal or Scotland.
You need the agreement of the other countries or the courts and each country is continually looking for the best deal for their own citizens.
The Scottish Government says it is confident on all counts and states its position is in line with legal advice. It states we will be in the EU, negotiating from within. We will not have to use the euro and we will be able to charge English students university fees.
But the fact is, at this stage we just do not know.