The reason why the grass always seems greener on the other side may be to do with chlorophyl and photosynthesis or other intrinsic physical qualities, but it's more likely to be caused by the medium-wavelength M-cones in the eye that are triggered by light in the 500-570 nanometre range.

If you were ever tested for deuteranopia, or one of the other kinds of colour blindness which prevent distinguishing red and green, you were probably given a set of cards showing circles made up of smaller, coloured circles, and asked whether you could see the numbers written on them.

I've often wondered whether some similar test could be devised for political colour blindness, given how party allegiance seems to make its subjects unable to see certain numbers which are as plain as day to everyone else.

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Take, for example, George Osborne's apparent inability to notice that the figures for both debt and public spending are rising. Or Ed Balls's failure to see that a restriction on a benefit payment is not a "bedroom tax", any more than a negative integer is a positive one. Or Alex Salmond being unable to read any figure except the one which claims that Scotland will be the sixth-richest country in the OECD. It would be uncharitable to single out any LibDem who can't see points until it's far too late.

Given this basic political fact – perhaps it's what George Bush senior meant by "the vision thing" – there is a limit to what can be achieved by drawing attention to objective reality. And while both sides in the referendum debate call constantly for clarification, specifics, accurate figures and so on (and are right to) there is a level beyond which such efforts are merely flogging a dead horse – or selling lasagne, as we put it these days.

If you are firmly convinced, and remain so despite any evidence put before you, either that Scotland is an oppressed nation which must be liberated, or that separatism will inevitably lead to the country falling into Zimbabwean levels of poverty and government mismanagement, then the numbers may as well not exist, whether you can see them or not.

For the majority of us, though, who are interested in claims from both camps, think that there are likely to be valid points on both sides and believe we can do our best to evaluate them objectively, there remains one obstacle to a clear-eyed view. It's the thing both the Yes campaign and the Better Together supporters claim they want to see more of: the positive case for the Union.

At the weekend, the Prime Minister said he wanted to "make the case for the UK" with everything he had, mentioned "unbreakable bonds between the people of these islands" and said: "Britain works. Britain works well. Why break it?" Yet the Deputy First Minister's instant response was to say that this was "an entirely negative attack".

It is as absurd to deny that there is a positive case for the Union as it would be to claim that it would be an impossibility for Scotland to go it alone. But the difficulty of a positive case for something which already exists is that it can always be presented as a negative case against something which doesn't.

If you imagine that everything will be better in an independent Scotland, you will see any mention of a common currency across the UK, freedom of movement and labour, equal access to benefits and services and so on as a threat that they may be lost if the UK breaks up. It is nothing of the sort.

It is a list of positive features which people are disinclined to notice because they have been evident for so long, like Poe's purloined letter, which was hidden in plain sight, or GK Chesterton's Invisible Man (who turned out to be the postie).

Take, for example, some things most pooh-poohed by the SNP – the idea that you won't be able to watch EastEnders, or that you'll need a passport to cross the Border, or that we'd lose the pound, or not be allowed to rejoin the EU. These seem unthinkable, and the Nationalists are no doubt correct to say it's overwhelmingly likely that they'd all happen.

The facts, however, are these: Scottish broadcasters would have to buy in programmes, or Scots subscribe to non-Scottish channels; EU rules for new applicants (which the EU says Scotland would be) might mean border controls, because of Schengen; at the moment they also mean agreeing to join the euro; if we did keep the pound, we wouldn't control the money supply; and you would have a different passport from your relatives in England.

But we don't need to put ourselves in a position where they might happen, and to say so is not to scaremonger. It is to identify positive features of the Union which would need to be clarified or resolved in the event of a Yes vote.

You may believe such fiddly objections are outweighed by the benefits of independence, or that they are easily surmounted. You may even be right. What isn't tenable is to say these points cannot be placed in the credit ledger for the Union. Anything which the Nationalists claim wouldn't change (because it is something Scots think of favourably) must be acknowledged as a benefit which we already enjoy by dint of our place in the UK.

It's a bit as if I were to say that my lot would be greatly improved by divorcing my wife, because I'll then be free to marry Angelina Jolie (or, under the new legislation, Brad Pitt) and live a life of globe-trotting glamour and luxury. All the evidence suggests this is a realistic prospect – I can't think of any reason why, as a Scot, with all the God-given advantages that implies, I shouldn't easily take my place at the top table at post-Oscars parties.

But if I then remember that I love my wife, and am very happy with my life as it is, you can't categorise my objection to breaking up my marriage as timidity about stepping into a bolder, better future. It may just be that I'm able to see what a lot I have going for me already.

Similarly, even if you think the UK is a doomed partnership and you want a divorce, you can't pretend that every point made in its favour is not a perfectly positive case, but prima facie evidence of negativity and timidity.