Henry Hill is a British Conservative and Unionist activist and writer, editor of Open Unionism, and writes Conservative Home’s Red, White, and Blue column every Wednesday

The makers of old maps, being creative souls, were loathe to leave the edges of the known world unadorned. Instead they populated the blank spaces with all manner of monsters. This was a very human thing to do, for fear of the unknown is a powerful force. One which Better Together, in a campaign strategy known fairly or not as “Project Fear”, have been using to good effect in recent months.

The SNP, unwilling and perhaps afraid to fight the referendum on nationalist principle, have sought instead to plot a clear and detailed map of what an independent Scotland would look like with their White Paper. The unionists in turn are taking pains to demolish each SNP pledge, scrawling over Salmond’s constitutional wish list a single, potent message: Here be dragons.

This must be frustrating for the separatists. The unknown is a battlefield that naturally skews in the Union’s favour, for whilst it’s true that we have no clear picture of what the UK will look like in the aftermath of a No vote, the UK already exists and Scottish voters of all persuasions are familiar with it.

The nationalists appear to have clocked that “fear of the known” is not a promising strategy, so have instead tried to turn the tables. The first part of this strategy – “project reassurance” – centres on the attempt to persuade Scots that everything they like about the UK will remain when they’ve left it. At present the unionists are making short work of it.

The second is to persuade Scots that it is the post-No UK which is the unknown quantity compared to the independent Scotland of the White Paper. This was evinced last week by Nicola Sturgeon, Salmond’s right-hand woman, setting a very obvious elephant trap: a demand to “see the details” of the pro-Union campaign’s proposals for “devo-max” in the event of a No vote. The Liberal Democrats, alas, have walked straight into it.

The demand does have a superficial veneer of reason to it. If the SNP can spell out in detail for the people of Scotland what independence will mean in policy terms (notwithstanding independence being ‘bigger than the SNP’), why can’t the unionists do the same for a ‘No’ vote? All the main party leaders have conceded the likelihood of constitutional change, after all. Why not just spell out what that change will look like?

This is bait on a nationalist hook, but not everyone thinks the Lib Dems have erred in biting: writing in the Spectator, Hamish Macdonell claims that Ming Campbell “knows how to kill off the Scottish nationalists”. This strikes me as implausible, not least because “killing nationalism stone dead” was what the original tranche of devolution was supposed to achieve back in 1998, and claims of devolution’s anti-nationalist lethality grow more threadbare with repetition.

In my view, spelling out a specific constitutional offer to Scots before the referendum is a very, very bad idea. It’s difficult to over-emphasise how bad an idea it is. But I’ll do my best: it will not only seriously hinder the operational effectiveness of the No campaign before the poll, but sow the seeds of future strife into a ‘No’ vote.

The appeal of the demand for the SNP is clear. If unionists don’t answer then the nationalists can raise the spectre of 1979, when Douglas-Home’s infamous promise of “better devolution” turned out to mean “eighteen years of integrated government by Margaret Thatcher”. It’s not clear how salient this line of attack might be with people who aren’t already nationalists, but the spectre of moderate Tories promising change and then electing a hard-right English fanatic holds an undoubted tribal appeal, especially with swing Labour voters.

But the advantages to the SNP of our attempting to meet the challenge are far greater.

First, any attempt to promise specifics will sunder the pro-union coalition. Better Together is greatly strengthened by being able to fight on the value of the existence of Britain itself, rather than constitutional details. The three main parties have just about managed to set aside sometimes deep-rooted enmities to stand together in their country’s defence. The prospects of that unanimity holding if a constitutional offer were required are close to zero.

‘Yes Scotland’ consists of the SNP and a couple of far-left minnows, so one party can effectively dictate its policy output. Better Together is a genuine coalition, and does not have that luxury.

Second, it will commit the grievous error of making the referendum a plebiscite on ‘our offer’ (if we can actually muster one), rather than ‘our country’. By taking up the line offered by certain devo-enthusiasts and casting a ‘No’ vote as a vote “for more powers”, we make any ‘No’ vote a conditional one. We deny people the opportunity to cast a positive vote for Britain itself – an eventuality we fought hard to avoid when we forced a two-option referendum.

This means that any subsequent deviation from whatever we cobbled together before September will, the nationalists shall doubtless argue, invalidate the ‘No’ vote. That notion of settling the constitutional question for a generation, if not forever? Forget it. There will be a period of acute misery whilst we discover that three very different parties cannot, in fact, cleave to a constitutional settlement agreed in haste in the white heat of the campaign, and then separation will be right back on the agenda. Ming’s plan to ‘kill the nationalists’ is a lifeline, cunningly disguised as a noose.

That is presuming, of course, that the major parties actually had the right to make a constitutional offer to Scotland without reference to the rest of the United Kingdom. Until Sir Ming’s speech, I was under the impression that a post-referendum constitutional convention, bringing together all parts of the Union to achieve a balanced and lasting settlement, was on the road to becoming conventional thinking amongst the majority of the pro-Union camp. Apparently not.

Simply put, you cannot have a unilateral referendum on devolution, an issue on which the rest of the UK rightly has a say. The nature of the British constitution post-No is for the British to decide, together. This is especially the case when the radical nature of certain proposals. These include devo-max, an impractical disembowelling of the UK so far-reaching that even early proponents like Murdo Fraser are backing away, and in particular Gordon Brown’s new notion of making the Scottish parliament “indissoluble”.

This would either be a meaningless sop, since one parliament cannot bind another, or it would be the end of the sovereignty of parliament and a more profound change to this country’s constitution than any yet passed. In a world where we (rightly) had a referendum on changing the voting system, the idea that the profound shift from a unitary to a truly federal state can be made by politicians without reference to the British (British) people is risible.

This latest proposal looks to be a repeat of the key mistake unionists have made throughout the process of devolution: storing up trouble for tomorrow by dodging battles today. It ought to have been made very clear to Scottish voters, long before now, that this is a referendum on the fact of Scotland’s membership of the Union, not the terms. That is all that a Scottish referendum, on a Scottish timetable and a Scottish question, could ever be.

A ‘No’ vote is not a vote for an unknown quantity. It is a vote for Britain. You might not know the exact shape of the Britain you’ll get if you vote ‘No’, any more than you know the exact shape of the Scotland a ‘Yes’ would produce, but you know enough to choose. Great Britain: in or out?