Conventional wisdom says soft Brexit is good for the union. If Theresa May steers the UK out of the EU but retains access to the single market, maybe a bit of freedom of movement it would make many Scots – the majority of whom voted to stay in Europe – think twice before voting Yes to independence.

After all, they’d be turning their back on some access to the EU for an uncertain future as an independent country who would still have to negotiate it’s way back into the club.

Conventional wisdom is wrong.

The reason Nicola Sturgeon is hell bent on keeping Scotland’s access to the single market is because a hard Brexit is bad news for the independence cause.

Never forget that while the SNP may claim to be Scotland’s party it is in fact a single issue movement focused on one goal only – independence.

If Sturgeon is opposed to the increasingly likely scenario that sees the whole of the UK crash out of Europe swapping single market access for full immigration controls, it’s because first and foremost it’s bad for her cause.

For if there is to be a hard Brexit, Sturgeon would have to sell the prospect of Scotland leaving the UK, joining the EU and being confronted with not just border posts for anyone wanting to travel south but tariffs for anyone wanting to trade with England.

She’d have her work cut out.The UK is a significantly more vital trading partner for Scotland than the remaining 27 countries of the EU. Scotland’s exports to the rest of the UK outstrip what it sell to Europe abour four to one, and it’s estimated that while 250,000 Scots jobs are tied to the EU, a million more rely on being in the UK.

It’s why Sturgeon for all her fighting talk is trapped. If there is to be a hard Brexit she needs to get Scotland out of the UK before the reality of that dawns. That’s looking like a two-and-a-half year window.

But the polls are stubbornly static.

She can’t have another referendum unless she knowns she’s going to win it. For to lose two votes on the same subject – and her draft legislation published last week suggests she’s going for the same question but banking on different arguments – would provide a definitive answer, closing the issue down for a generation for real this time and begging questions not just about what next for the SNP but what’s the point of the SNP.

With Yes still hovering around the 45 per cent mark in current polls Sturgeon needs to add a good 15 per cent before she can consider triggering indyref2.

Now, some of her supporters point to the last independence campaign when support for the proposition rose from a historic position of around 25 per cent to 45 per cent by polling day. They claim the same can be done again.

But that was a long campaign and Sturgeon does not have the time, never mind the fact that most of the soft Yes vote has been hoovered up now and convincing those that remain will prove much harder.

And, according to my Number 10 source, Theresa May knows all this.

That’s why she can dismiss Sturgeon’s bleating. Why she can sit around the Cabinet table with her as she did yesterday and, despite promising respect, actually give her short shrift.

May’s in the stronger position on this one. She’s newly installed, and confident that she can go to the country and win at will.

Sturgeon’s overseeing an increasingly tired SNP administration (albeit, like May, there is no credible opposition to speak of). If she doesn’t deliver independence it’s not just her political career but the future of her entire party that would be pitched into the balance.

Unlike David Cameron, May has no specifically Scottish special adviser and her dismissive tone towards Scotland has led some to speculate that she doesn’t get it or doesn’t care.

Quite the opposite.

Whatever other drawbacks, hard Brexit brings it is the most sensible position to take if your number one priority is keeping Scotland in the UK.

In the absence of any evidence as to what else her strategy may consist of, perhaps that is May’s game.