Tomorrow the Prime Minister will make her “big speech” on Europe, which after weeks of “Brexit means Brexit” vacuity finally promises to give much-anticipated detail of the approach she intends to take in negotiations with Brussels.

According to reports over the weekend, this will likely refer to the UK making a “clean” break with the institutions of the European Union as well as forging a “strong new partnership” with the constitutional club it’s now on course to leave. Leavers will no doubt like the former message, while the purpose of the latter is to placate Remainers.

Basically, Theresa May is trying to keep everyone happy. “We need to get on and make Brexit happen,” she’s expected to say. “We need to put an end to the division and the language associated with it – Leaver and Remainer and all the accompanying insults – and unite to make a success of Brexit and build a truly Global Britain.”

It’s a noble ambition, and hardly novel in the political world, but it’s also unlikely to succeed. As the English poet John Lydgate said in words later borrowed by Abraham Lincoln: “You can please some of the people all of the time, you can please all of the people some of the time, but you can’t please all of the people all of the time.”

Like all good quotes it’s contested, but it captures the essential truth that politicians want to be popular, and the pursuit of that goal generally involves biting off more than they chew. There are Leavers and Remainers, and Yessers and Noers, and never the twain shall meet.

The same quote popped into my mind during Saturday’s impressively large gathering of the Scottish Independence Convention (SIC) in central Glasgow: everyone present agreed that independence was good and necessary, but beyond that there was predictable confusion about how to go about securing a Yes vote in a referendum the First Minister continues to insist is “highly likely”, just not anytime soon.

Some of those present argued for a campaign based on the “principle” of independence, in other words not via a policy prospectus but the argument that those best placed to make (all) decisions about how Scotland is governed are Scots themselves. Others argued for a more practical vision, i.e. one that spelled out in specific detail what would happen in terms of currency, public spending, taxes and so on.

The Common Weal think tank – with its quixotic commitment to put “all of us first” – falls into the latter camp, having recently produced a series of papers as part of its “White Paper Project”, basically an attempt to come up with a more convincing independence blueprint than that in 2013. The SNP’s ongoing Growth Commission also acknowledges the need for a detailed vision of what independence will look like in the context of Brexit, fluctuating oil revenue and unanswered questions about currency and the sizeable gap between what Scotland spends and raises in tax.

Several speakers at the SIC clearly believed that the SNP’s vision of independence, as expressed in the “Scotland’s Future” doorstopper, had dominated the first independence referendum rather too much, but here is where some delegates and panelists tied themselves in knots. Dr Craig Dalzell appeared to support the “principle” approach, declaring that independence was “not the end” but “a means to greater ends”.

Initially, Stewart Kirkpatrick seemed to agree, saying the question wasn’t “what bleeding currency” Scotland uses but rather that the decision should be “up to people in this country”. There were many different visions of independence, he added, and what was needed next time round were “different messages coming from different organisations”. “Let’s give voters a sweetie shop,” he ventured, “with a panoply of visions.”

But not only does this conflate principle and practice, it falls into the trap of trying to please all of the people all of the time. Furthermore, Yes Scotland more or less adopted that strategy between 2012-14 and ended up tying itself in knots. Sweeties, of course, aren’t necessarily good for you, and too many (most likely conflicting) choices just end up confusing the electorate.

On Saturday the singer Pat Kane spoke of uniting the “managerial class” with the “working class” in a common appeal for independence, but not only would such a platform be difficult to assemble, but someone’s going to end up disappointed. If, for example, an independent Scotland goes down the low-corporation tax small-state route then those who voted for a left-wing “vision” of independence are going to be left high and dry, and vice versa. As in 2014, voters won’t know where to look or who to believe.

This confusion permeates not only the Yes movement but the SNP and Scottish Government too, a quixotic belief that they can appeal to every voter in Scotland regardless of class, gender or ideology. At last year’s Holyrood election, a widely-distributed SNP leaflet asked “Who benefits most from our policies?” before giving the implausible answer “We all do”. In the new Scotland there shall be no losers, only winners.

The same balancing act can be seen in the Nicola Sturgeon’s increasingly convoluted approach to Brexit and another independence referendum. Conscious that a million Scots, included a big chunk of SNP supporters, voted to Leave, the First Minister has now abandoned the idea of keeping Scotland in the European Union (her stated aim a few months ago) and is instead pursuing the “compromise” of remaining part of the Single Market, an attempt, as with the Prime Minister, to keep both sides happy.

When it comes to Brexit this is not an ignoble aim, but when applied to public policy it becomes much harder to justify. Take the Scottish Budget, which is sort of increasing income tax while sort of keeping it the same. The upshot is that Scots who pay the 40p rate are irritated at having to cough up (slightly) more than their English counterparts, while those on the left of the independence movement grumble that it doesn’t go far enough.

Then there’s the Time for Inclusive Education (TIE) campaign, which has been lobbying MSPs to back legislation to tackle homophobic bullying in schools. The First Minister, who has a good record on LGBT issues, has expressed general support but seems reticent about going any further, most likely because compelling Scotland’s faith schools to implement such legislation would prove politically tricky.

For the past ten years the SNP in devolved government has worked hard to prove Messrs Lydgate and Lincoln wrong by keeping all of the people happy all of the time, but there comes a point when a party of government, particularly one committed to delivering independence, has to pick a side. At some point the alternative, endless fudge and prevarication, will end up pleasing fewer and fewer voters.