According to some dodgy pseudo-scientific algorithm, today is the most depressing day of the year. But do not be sad because it’s also the day Gordon Brown is making another speech about the constitution. The UK needs to change if Scotland is to remain a part of it, he says; either we remake Britain or we lose it. You're feeling better already, aren’t you?

The thing is, the speech will probably be quite good. In 2014, just before the referendum, I watched the former PM speak to a group of voters in Ayrshire and it was an impressive performance. Some mock Brown as the last of his kind, and yes, there was something of the white rhino about him as he stomped about the stage. But he tapped into quite a few things that were critical to the debates about Scottish independence and Brexit – and still are.

One of them is a principle that many No voters and Remainers work to, which is that sharing and co-operation are better than separation and splitting apart. If a lot of the debate over independence is governed by instinct – and I think it is – then the instinct for co-operation is one of the strongest, just as other people feel an instinctive pull towards nationalism and separatism. Often, our economic and political arguments are just post-facto justifications for our instincts: we look for facts to justify how we feel.

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Brown also put a lot of emphasis in his 2014 speech on the need for the Union to change. Again, some people have mocked him for this, but he said that a No vote was a vote for increasing the powers of the Scottish Parliament and it’s a theme he’s returning to in his speech today. This is absolutely central to the debate over independence: many Scots feel strongly about their British identity but also like the idea of a progressive, federal UK. They want to stay the same – British – within a changed system.

Changing the UK in this way has the potential to affect the SNP’s momentum. A federal system based on proportional representation would mean less chance of a Tory government at Westminster and so, by extension, less difference with Scotland. To that extent, federal reform might also undermine the case for independence by making the UK more responsive to Scottish opinions. This will never be a factor for the all-or-nothing separatists, but it could be for the more reasonable switherers in the middle.

Aside from changing to a federal system, there a number of other factors that could affect the SNP to a greater or lesser extent and one of them is fatigue, which happens when politicians say things that are contrary to the voter’s experience. One example would be a Tory minister saying employment has never been higher when voters can’t find a decent job - and the same applies to the SNP. Nationalist ministers say a second referendum will happen this year, but voters see that it won’t. They can also see ministers starting to squirm their way off the hook by switching the emphasis to the 2021 elections. The result is that a kind of fatigue sets in and our faith and support starts to flag.

Another important factor, after change and fatigue, is events and it’s potentially the most important of all. Much of the attention independence still gets is because of constitutional events: stuff that’s happened. The EU referendum. The win for Leave. Boris Johnson being elected Tory leader. And Boris Johnson winning the election. Leaving the EU may be another of these events but, with trade deals, general elections and referendums a long way off, I suspect the events are going to run out a bit, unless the SNP can win next year’s election outright.

Which takes us to the last, and perhaps most perplexing, factor, which is experience. I was having dinner with some friends at the weekend – friends who have often voted SNP. However, one of them was telling me she almost didn’t vote for the nationalists this time because of her experience of education. In her view, her kids have suffered in school from a lack of resources, a lack of emphasis on success, and a lack of focus because of the Curriculum for Excellence. I hear the same from friends who are teachers and you can interpret this in one of two ways: either (amazingly) voters are still supporting the SNP even though their local schools are in a bad way, or (finally) the voters’ daily experiences are starting to turn them off the SNP and the effects will soon be felt.

The reason the experience factor is perplexing is that it hasn’t taken effect sooner. Education and health have been in a mess for ages and yet support for the SNP remains high. Traditionally, the experience factor is critical to the outcome of elections, but the primacy of constitutional affairs has meant people make their political decisions based on other factors. However, open the book of British political history at any page and it reads “success does not last forever”. Even the SNP will not be able to rewrite that one.

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All of these factors operate on their own, but they also depend on what the UK Government does now and what we hear from inside Number 10 is that a report has been commissioned from advisors on how the Union can be supported. A unit has also been established to develop policies that might keep Scots on the side of No.

With any luck, sooner rather than later, this may lead to the PM realising the importance of the change factor in particular. You can try to influence events; you can try to talk about the SNP’s poor record in government; you can even spend more money in Scotland (and there will be some of that). But ultimately, you’ve got to live up to the trends that Gordon Brown talks about. Many Scots believe instinctively that sharing is better than separation, but the instinct has to be reflected in the system - fairly, openly and widely – and that’s going to mean some pretty fundamental change.