SOMETIMES, opposites can mean the same thing. Take Willie Rennie and Rona Mackay for example.

Mr Rennie, leader of the Scottish Lib Dems, said at the weekend that a second independence referendum should be blocked even if the SNP and Greens win another majority at Holyrood. In response, the nationalist MSP Ms Mackay said recent events had boosted the case for Scottish self-government. On the face of it, those two statements look like very different opinions – opposites in fact. But could they be pointing in the same direction?

I think they could, and happily you don’t have to take my word for it – take the word instead of my distinguished witnesses: Richard Dawkins, star of evolutionary science, and Kenneth Williams, star of Carry On Up the Khyber. Over the last few days I’ve been reading Mr Dawkins’s superb new book Outgrowing God; I’ve also been dipping into the diaries that Kenneth Williams kept for most of his life, and both books – though very different obviously – have got me thinking about how opinions work, how they change, and how and when they should influence the way we live.

The first thing to say is that there is usually a “public opinion” (i.e. a majority opinion) about most of the issues of the day – in Outgrowing God, Dawkins uses a surprisingly unscientific phrase to describe it: “something in the air”. Obviously, this “something” is not static; majority opinion can change over time. But on the subject of the constitution it’s fair to say that in the 1970s, 80s, and arguably right up until this year, the something in the air was that Scottish independence was not what most people wanted.

You can see what I mean in Williams’s diary for Friday 2nd March 1979, the day after the referendum on Scottish devolution. “The TV news announced that Wales said no, and Scotland only got 32% for,” he wrote. “So that should effectively put an end to the whole stupid charade … I opened a bottle of champagne.”

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Of course, the 32 per cent figure Williams refers to is only part of the picture. Of those who voted in 1979, 52 per cent supported devolution, but the law said it would only go ahead if there was support from 40 per cent of the eligible electorate and, on the day, the turnout was 64 per cent. That meant the proportion of the registered electorate that voted yes was only 32 per cent, as Williams said. And so, the proposal failed.

Forty years on, I don’t think anyone would suggest a repeat of the 40-per-cent rule, but what Williams wrote in his diary in ‘79 reflected the prevailing views at the time. The “something in the air” was that independence was a minority obsession; there probably wasn’t even a settled, big majority for devolution either.

It’s important to remember too that the situation stayed that way for a long time. I remember speaking about this subject to the former Scottish Secretary Malcolm Rifkind and I asked him why, as a supporter of devolution, he hadn’t fought for it harder in government. His response was that it wasn’t a dominant issue. If it had been, he said, why weren’t there thousands of people marching down Princes Street demanding a Scottish parliament?

I think that’s a fair point, but the question now is: how much have opinions changed and what should we do about it? Obviously, we do now have thousands of people who march for constitutional change, and it’s fair to say there is “something in the air” in the way there wasn’t in the past. In his book, Dawkins explains the “something” process in more detail and says prevailing opinion changes because of ordinary conversations, in cafes and pubs and around dinner tables. In that respect, it is similar to Darwin’s evolution, except that it’s an evolution of opinion and thought rather than sinew and muscle.

I’m not going to pretend I have scientifically robust evidence to back up Dawkins’s idea, but I think I may have seen some of this philosophical evolution in some of the conversations I’ve had myself. I’ve heard people who voted no in 2014 tell me they might vote differently next time because of Brexit and to that extent they prove Rona Mackay’s view that recent events have boosted the case for Scottish self-government in the minds of some people. Some of the opinion polls would also seem to demonstrate the same point.

But the question is: what should we do with that information and how important should it be? What Willie Rennie said at the weekend was that, even if there was a pro-independence majority at Holyrood, it would be legitimate for the UK Government to reject a second Scottish referendum partly on the grounds that big-bang constitutional change is chaotic, damages the economy, and divides the country.

I think what’s happening with Brexit proves Mr Rennie’s point about division, but he’s also right to question the idea that a simple majority at Holyrood represents a mandate for constitutional change. Much of the present division and chaos Mr Rennie describes is because of the closeness of the result of the EU referendum – Leave attracted only 51.89 per cent and yet it is now being portrayed as a mandate for a no-deal Brexit. The 2016 Scottish election was similar – the SNP attracted only 46.5 per cent of the constituency vote and the Greens 0.6, all on a pathetic turnout of 55 per cent, and yet that is portrayed as a mandate for a Scottish referendum.

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The obvious answer is to base constitutional change not on what is effectively the views of a minority, but on a reflection of the settled will of parliament and the public. In the US, constitutional change needs support from two-thirds of the Senate and the House of Representatives; there’s also a strong argument for referendums to require a two-thirds majority of those who vote. And there’s much to be said for the idea proposed at the weekend by Alan Renwick, of University College London, that Scotland should only become independent if voters say yes in two referendums.

Scottish nationalists, Brexiters, and anyone else who wants a referendum will resist such changes, and they’ll do so for an obvious reason: they want it to be as easy as possible to win. However, shouldn’t profound constitutional change only happen if it genuinely reflects what most people want? Willie Rennie thinks a thin majority by two parties in the Scottish Parliament would not pass that test. I think democrats should agree with him.