WHEN Scottish voters go to the polls in the autumn of 2014, they will want to know what they are voting for.

Ever since the subject of EU membership was first raised by Labour MSP Catherine Stihler in May 2011, Scottish Government ministers have conveyed the impression that continued membership is a given. Deputy First Minister Nicola Sturgeon has insisted that it is a "cast-iron position" that an independent Scotland would not have to rejoin the EU or adopt the euro. That certainty now looks far from convincing. The situation became yet murkier yesterday following an article in the Spanish newspaper El Pais suggesting that European Vice-President for Justice Viviane Reding believes an independent Catalonia would have to re-apply to the EU, even if the separation was by mutual consent with Madrid.

If that is the case, it carries clear implications for Scotland in the event of a Yes vote in 2014. Alex Salmond must have been glad that he had another engagement when Labour raised the whole issue of his handling of this business at Holyrood yesterday and demanded a judicial inquiry. Once again Ms Sturgeon was left to defend her Government's position, insisting there was no contradiction between Mr Salmond's statements about ministers seeking legal advice and her own admission last week that they had done no such thing.

The only thing that is clear about EU membership in an independent Scotland is that it is unclear. (A recent conference at Edinburgh University came to the same conclusion.) It is unlikely that Scotland could not be an EU member if it asked to be but there are issues about timing and terms. At least one of the SNP's backbenchers appeared to concede this yesterday.

This is a real challenge for the Yes campaign because this issue is like a court case in which different standards of proof apply to the two sides. While the independence lobby needs to provide incontrovertible proof of its contention about uninterrupted membership, its opponents only need to demonstrate reasonable doubt to inflict serious damage on the Yes campaign.

The SNP has sought to close down debate on this issue by implying that ministers had been advised that there is nothing to worry about. Now that doubts have been raised, the longer that Mr Salmond allows doubts to persist, the bigger the stick it hands to his opponents to beat him with. Continued silence on this issue would imply that he has not received the advice he was hoping for.

Of course, given the depth of the eurozone crisis, by the time any such negotiations take place, they may have been overtaken by events. The euro may have been consigned to history and the EU itself may be facing an existential crisis.

However, the SNP cannot rely on this, which is why it is in ministers' own interests to settle this question as best they can. Otherwise, many of those currently wavering will opt for no change over a leap into the unknown.