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And so, from the boss of BP, further questions concerning the big question, to be addressed by the people of Scotland in September. And once again, it is the elements of interdependence - rather than independence - which are prompting debate.

Bob Dudley of BP speaks of uncertainties surrounding the referendum offer. Specifically, he talks about a "question mark" over the currency to be used by an independent Scotland and the pace of settling potential membership of the EU.

It will be noted that both these issues require agreement from power blocs beyond Scotland. It will be noted that, consequently, advocates of independence are unable to offer an unalloyed, unilateral guarantee on either point.

Not unwilling. Unable.

That is because sharing the pound post independence, as part of a sterling zone, requires the remainder of the UK to assent. That is because membership of the EU and the concomitant terms require negotiation with Brussels and agreement from existing EU member states.

Debated endlessly

The response from Scottish government ministers is to argue that it would be in the interests of the rest of the UK (rUK) to share the pound - and in the interests of the EU to retain the people of Scotland in membership.

On the Nationalist side, there is apparent exasperation at issues being raised, again, which they regard as having been debated endlessly and, they would argue, settled.

On the Unionist side, there is evident delight that a big name business leader has voiced concern - while simultaneously stressing that BP was continuing to invest substantially in Scotland.

Those campaigning to retain the Union believe that the weak points in the Nationalist case lie with those interdependent issues - because, by definition, they feature intrinsic uncertainty for the very basic reason that they are not decisions to be taken by one party.

Now, are the Unionist side talking up the doubts to assist their case? Of course they are. They now hope that other business leaders will express concern.

To be clear, Mr Dudley's view is not uniformly shared across the business sector. Many, including senior figures, have argued that independence would enhance the Scottish economy and give Scotland a distinctive marketing pitch.

Significant role

But others will share Mr Dudley's doubts. To date, relatively few business leaders have come forward.

In the 1979 referendum, the business sector played a significant role in casting a shadow of doubt over the then proposals for devolution.

By 1997, it appeared evident that the revised plan for devolved self-government had substantial popular support - as did the newly-elected UK Labour government which was advancing the scheme.

Cautious business leaders could see that the scheme was going ahead, that the Tories had been heavily defeated - and that, for the foreseeable future, they would require to work with Labour. Those few who voiced doubts were chided, bluntly, by John Prescott.

Similar pragmatism may be at play now, at least in Scotland. Some business leaders may be reluctant to speak out too bluntly for fear of upsetting the incumbent Scottish government. Even Mr Dudley modulates his remarks by stressing the solidity of his company's presence in Scotland.

The points raised by Mr Dudley are not new. But, once more, they will drive the two sides to respond. Better Together will talk up the doubts. Yes Scotland will seek to offer reassurance. Doubt and reassurance.