Methods of gaining independence include self-determination, a referendum without UK approval, legal proceedings, consultation, and other forms of negotiation.
There are four conditions.
First, the referendum question must be clear and unambiguous. Evidently this was the case first time around. There may however be an issue this time. A referendum tends to narrow an issue down to a binary choice: yes or no to independence. In the ordinary run of things, it would be preferable to have a clear alternative to independence to hand when a referendum is held. However, this alternative vision is yet to be defined.
Second, the referendum process must be open and inviting to all sides. Where this second condition is fulfilled, the unionists cannot obstruct the referendum through a boycott. The vote will count, even if they chose not to participate, unless total voter participation falls below a reasonable percentage.
Third, the result must be clear. In most cases, this has required a simple majority of 50 per cent plus one vote.
And fourth, even if the pro-independence side wins, it cannot simply walk away and declare independence. It, too, must take account of the interests of the other side and negotiate in good faith about a balanced and reasonable divorce settlement.
In the case of Scotland, extra caution will be required. Scotland can only achieve its goal of independence within the European Union if it is seen to have gone through an entirely consensual process of self-determination, including a divorce among peaceably consenting adults. Otherwise, it is virtually guaranteed that the members of the EU opposing any case of secession as a matter of principle will veto Scotland’s application to join the EU.
In short, whatever the rhetoric of the upcoming Scottish elections, both sides will in the end need to engage with one another with respect, in a spirit of accommodation and compromise.
It will not be possible to deny a referendum indefinitely. On the other hand, Scotland can only succeed as a state within the EU if it can demonstrate that it has obtained the fullest consent from Westminster for its potential journey towards independence. This gives both sides equal weight in the discussions that will follow upon the elections in May.