Did you ever see a terrible film called Next, about a man who could see everything which might happen in the next couple of minutes, then rewind and choose the best option?

It starred Nicolas Cage, naturally. Cage isn't always terrible - he can indeed sometimes be quite good - but the fact remains that if you wanted a film to be a stinker the first thing you'd do would be to cast him in it.

The second step the filmmakers took to ensure it would be dreadful was to base the film on a short story by Philip K Dick. I'm evangelical about the late PKD, since I discovered that he was one of the great writers and visionaries of the last century while he was actually still alive. Back then, having written dozens of ­brilliant novels and hundreds of short stories, he was living off cat food and amphetamines in penury.

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After his death, however, movies based on his work - including Blade Runner, Minority Report, Total Recall (twice), The Adjustment Bureau and half a dozen others - made billions of dollars. Alas, these films are seldom any use, and on the rare occasions that they are, not half as good, or one-tenth as clever, as the books on which they are based.

Anyway, had the producers of Next possessed the talent on which their film was predicated, and foreseen the critical and commercial reaction, they wouldn't have bothered making it. But they hadn't, just as I can't tell you how Hello Beautiful will do in the 2.10 at Hamilton this afternoon.

I take this roundabout way of ­stating the obvious - that we can't know the future - to point out that even our best guesses at it are often lousy. Nobody would have thought that a drug-addled religious nut who wrote philosophy disguised as dimestore science fiction would become the hottest property in Hollywood. Once his stuff started hauling in cash by the cartload, it no doubt seemed sensible to cast a big star in an ­atrocious $70 million film based on one of his stories.

When it comes to the future, nobody can plot out every possible option, and the infinite variations of every subsequent scenario, though we'd all like to be able to. Which is why yesterday's YouGov poll, which found that 56% of voters would like the political parties opposed to ­Scottish independence to set out their plans (if they have any) for further devolution now, isn't very startling.

It is, however, pretty stupid. The referendum question is a guess about the future, but it has the virtue of being a straightforward one: should Scotland be independent? There are lots of subsidiary hypotheticals which follow: we'd be richer, we'd be poorer, we'd be in the EU, we'd be chucked out of it … and so on, ad infinitum.

For some people, the vital question will be whether we'd really be better off (either materially, or in the way we felt about our political priorities) in an independent Scotland. For others, it is whether you want to ditch the UK's shared history and traditions and make a foreigner of your Auntie Jessie who lives in Lancaster. Either way, though, it's a clear question - do you want Scotland to be ­independent in the way that, say, the Republic of Ireland or Norway or Iceland are?

The idiocy of demanding in advance the same thing of proposals for further devolution, should we vote No next year, is that nobody could spell them out, or secure agreement on them, even if they knew what they might be. The YouGov poll was commissioned by the lobbying group Devo Plus, so it unsurprisingly asks whether people would be more or less likely to vote No if they knew the Scottish Parliament would then get the sort of powers outlined in that organisation's paper Devo Plus: A New Union, published last year.

The trouble with that is that it's like asking whether Hello Beautiful will win the 3.40 at Brighton next October if it finishes third this afternoon, or whether one of Philip K Dick's short stories would clean up at the box office if it were filmed by Quentin Tarantino with Peter Capaldi in the lead role. None of that stuff is on the table.

It's not that there's anything ­intrinsically daft about the Devo Plus proposals, which include the permanent right to legislate on non-reserved powers and the aim that Scotland should raise the majority of the expenditure recognised by statute. And they have the undoubted merit that they seem - on the basis of ­polling - to be the most popular option when Scottish voters are asked what they'd like.

Received wisdom and opinion ­polling, however plausible, cannot be taken as "the settled will of the ­Scottish people", though, especially when it is for a grab-bag of possible options. That's why the No campaign can only really be about saying No (even if, as can hardly be avoided, it risks looking negative when it does so). It can't be about saying: "No to independence, but Yes to this other measure, and possibly this one, though we can't agree about it, although perhaps we could add that one over there if you fancy it."

In any case, whatever those suggestions might be, they are hardly detailed, costed, thought-through policies. How could they be? And if the opponents of independence (rightly) question the optimistic assumptions of some Nationalists, they can hardly present as a more realistic option even less worked-out proposals.

There are other reasons why it's not a reasonable alternative. The Unionist parties don't all subscribe to the Devo Plus agenda. Indeed, there isn't even uniform support for its ideas within any one of those parties, though they all include plenty of people who might well like some of them.

Then there are the numerous ways in which the current settlement might be thought deficient, even if you're resolutely opposed to breaking up the Union. Why is broadcasting a reserved power? Why should Holyrood demand additional tax-varying powers when it has never used the ones it has? Why shouldn't any inadequacies in the current arrangements indicate that devolution was a bad idea in the first place, rather than make us assume we haven't had enough of it? Or is it just that the whole of the UK needs to be federalised for it to work? And when is somebody, for crying out loud, going to try to answer the West Lothian question?

Proponents of Devo Max are, to their credit, attempting to grapple with such questions, as are so many of us, in our idle moments. But it's pointless to insist that they should be presented, in advance of the referendum, as alternatives to a Yes vote, as if there were universal agreement on what the answers were.

The reality is that, even if most of us want these things clarified, or some more powers, we wouldn't all come up with the same checklist. There are enough "known unknowns", as Donald Rumsfeld once described questions which one could anticipate but not answer, about the referendum as it is worded. It isn't really useful to try to second-guess a whole lot of unknown unknowns, even if it might affect how you'd answer the first question.