Guards at the border:

that phrase has long been used as shorthand for hysteria in the independence debate, because it seems so unlikely. After all, Ireland and the UK are separate sovereign nations and yet there is free movement between them within the Common Travel Area of the British Isles. Provided any future independent Scotland were part of the Common Travel Area, as the Scottish Government would like it to be, why would there have to be border posts at Gretna? Any politician making that claim runs the risk of being seen as scaremongering.

Step forward Scottish Secretary Alistair Carmichael who has waded into that territory with a warning that there is a "real danger" the UK Government would have to put guards at the borders. His concerns focus on immigration and the fact that if Scotland became independent it would have a substantially different policy from the rest of the UK. Ireland, he suggests, has kept a "broadly similar" immigration policy to the UK in order to ensure it remains part of the Common Travel Area.

Is this scaremongering? Yes, in the sense that it is designed to spread anxiety about what a post-independence Scotland might look like. However, it is not entirely without grounds, just unlikely. It is just about possible that a UK Government could fear the ingress of illegal immigrants through Scotland and choose to set up border controls. The Scottish Government has dismissed the possibility, but in truth neither side can be categorical. The Scottish Government states it will not happen because it will be in no-one's best interests, but it does not know the mind of a future UK Government.

There is one rather significant flaw with Mr Carmichael's logic, however. If Scotland had the more liberal system, would immigrants not wish to come here instead of England? It is not quite as simple as that since those denied entry to England and Wales but wishing to join family or friends south of the border might enter through Scotland and head south, but it seems likely Scotland's more welcoming immigration policy would draw at least some of those who might otherwise have headed for the rest of the UK.

The LibDem Scottish Secretary is part of a coalition with the Tories, who are pulling to the right on immigration. With these comments, he risks being seen as defending an ever tougher UK policy that does not suit Scotland. Yet he should also consider that this worst-case-scenario style of anti-independence message tends just to put voters off.