Next week will be the second anniversary of the 2014 independence referendum, the defining moment in modern Scottish history, but we're no nearer understanding when the next one will be. One day Nicola Sturgeon seems to be saying it is highly likely and she's itching to call it; the next the option has retreated again into the optional never never. In her Programme for Government, the First Minister outlined the legislative process for another referendum, arousing expectations that she was preparing to take the plunge.

However, she has merely put the draft referendum bill out to consultation, where it could lie until the next election. Her caution is in many ways commendable, especially given the lack of clarity over the meaning of Brexit, which is rivalling Fermat's Last Theorem as one of the great head-scratchers of the age. But it is also beginning to sound a little like indecision. There is a limit to how long she can keep the nation in this state of "referendus interruptus". Everyone has been criticising Theresa May for not having a clue whether or not Britain should remain in the single currency but Ms Sturgeon seems to be no stranger to uncertainty herself.

Or is she? I think after this week the odds may have moved decisively against a referendum happening, as many in the SNP seem to believe it will, during 2017. I don't know where this rumour started but it has built such a head of steam that many in the independence movement are convinced the campaign has already begun. Yes groups are getting organised, crowd funds are being rolled out and demonstrations are planned. It's going to come as a great disappointment to many if, as seems likely, the referendum is put off until 2021 at the earliest.

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Why do I say that? Well, the surest sign was the announcement on Monday that the First Minister is seeking to form a “coalition” with ministers in the UK cabinet who want to keep Britain in the European single market (ESM). She made clear that she would only call a second referendum if this coalition failed. Of course, she hasn't actually ruled anything out but if her stated aim is to keep Scotland in, she can't reasonably call a referendum until it is clear that the UK is out.

She is therefore putting the most crucial decision about Scotland's constitutional future into the hands of some of the flakiest political personalities in UK politics, including Boris Johnson, who is one of the cabinet ministers thought to believe that Britain can retain privileged access to the EU single market.

The other prominent pro-single marketeer is the Chancellor, Philip Hammond. I can't really see the “45” group of ardent Nationalists being happy leaving Scotland’s destiny in the hands of the Tory politician who said in April 2014 that a Yes vote would make Scotland vulnerable to “attack from space”.

Of course, it may be that Ms Sturgeon is playing a clever game. She may already have concluded that the single market is a busted flush. There are many reasons to suppose this might be the case. The Minister for Brexit, David Davis, said it would be “very improbable” that Britain could remain in the single market because it would mean Britain losing control of its borders even before it had recovered them. Members of the single market are required to observe the EU's “four freedoms” for goods, capital, services and people to move freely. It's the last one, immigration, that is the red line for Brexiters.

Theresa May reportedly slapped down her Brexit minister for making this statement. She is not ready to rule out Britain retaining some relationship to the single market. There is an arcane debate in Westminster about whether Britain could still have "access" to the ESM without actually having to be a member. Some Brexiters believe Britain could secure all the preferential terms of trade in the negotiations. The argument is that Europe will have to do some deal as it exports more to us than we do to it.

But the EU doesn't really work like that. There are 27 countries remaining in the EU and not all see their interests as the same as those of German car manufacturers. The single market is a geo-economic entity whose first responsibility is to strike deals, for its members, with the rest of the world. It isn't going to negotiate on behalf of non-members. And as the Australian government made clear this week, while it is negotiating with the EU, it will not be prepared to enter into any trade deals with Britain independently of the EU. It’s the Brexit shut out.

Britain could have crude access, after Brexit, to the single market, but it would be doing so along with all the other nations of the world. Anyone can trade with the EU provided they accept its terms. Haiti has access to the single European market but it has to negotiate tariffs and conditions. If Britain is out of the single market we can't expect the EU to negotiate preferential deals with the rest of the world on our behalf. We have to do this on our own. The Scotch Whisky Association believes that this could be costly and a 20 per cent tariff could be slapped on Scotland's national drink by countries such as South Korea and South Africa.

It is a mess. But there is not going to be any finality on the single-market issue for some considerable time. Indeed we won't know the shape of the Brexit deal until 2019 at the very earliest, too close to the 2020 General Election to hold a referendum. The only certainty in the meantime is that Scotland cannot conduct any significant diplomatic activity independently of the UK while negotiations continue. The EU only recognises the UK as the member state, and regards Scotland as a region, which it is.

This is why some in the independence movement think Ms Sturgeon shouldn't get caught up in the Gordian Knot of trade and should step aside from the Brexit imbroglio. Why shouldn’t Scotland just get out of the UK now and start forging its own relationships with Europe? By hitching the fate of Scotland to the soft Brexiters, Ms Sturgeon risks losing the initiative. The momentum of the independence movement could be dissipated in the complexities of non-tariff barriers, services passports and multilateral trading preferences. This is death to an independence movement that is a simple binary choice: in or out, yes or no, stay or go.

The polls suggest Yes would lose another independence referendum. But that doesn't discourage the go-for-it tendency. The last independence referendum began at 30 per cent and ended at 45 per cent. A second referendum would be campaigning from a much higher base of around 47 per cent, they say. It’s in sight. But if my impressions are correct, they have their work cut out persuading the SNP leader to set aside caution. Right now, she's not having it.