I have never been altogether persuaded that David Cameron is good at making speeches.

Of course, he's a better speaker than Ed Miliband, but then so is almost everyone, including the cast of The Artist and the dog that used to say "sausages" on That's Life.

The consensus that it is one of Mr Cameron's strengths can be dated precisely to the address he gave to the Conservative conference in October 2005. My impression at the time was that it was exactly the sort of speech you would expect from a former director of corporate affairs for a television company who had also made a close study of the rhetorical techniques of the Blairite wing of the Labour Party.

Though I struggled to see that much was being said, it was clear that aspect of it didn't trouble lots of people. As a matter of empirical judgment, since Mr Cameron went on to win the party leadership and become Prime Minister, it would be silly to deny that it was effective.

Effectiveness will ultimately, of course, also be the test of his address in Edinburgh last Thursday, which many commentators have decided was another good speech. Most of what he said was unexceptional and unexceptionable – unless, as is always the case, you happen to be a Nationalist with a modem – but it was tactically astute.

There has never been any mileage in Unionists attempting to deny that Scotland could be viable as an independent country. Nor ought there to be any doubt that it would happen if a majority chooses it. Sensibly enough, the Prime Minister didn't try either of those.

Instead he laid it on thick. Nursery of literacy - champion of liberty - Enlightenment - turbine hall of the Industrial Revolution - powerful modern economy - blah, blah - Adam Smith - David Hume - rhubarb - Lord Reith - (grits teeth) Keir Hardie - Donald Dewar - (takes out small onion) heroic soldiers - Third World aid - my father's father was a Cameron – well, yes, we rather expected as much. The line about Aberdeen having the same number of universities as England by the 16th century is always a crowd-pleaser. One assumes that Glen Michael and his Cartoon Cavalcade were left out only because some sharp-eyed adviser remembered that he had been born in Devon as Cecil Buckland. All the same, this feel-good hokum has a purpose.

It is unfortunately necessary if you want to ask whether Scotland would be better, and better off, outwith the Union and not get accused of scaremongering. It doesn't seem especially timid to ask for a few details to be clarified: points such as what proportion of oil revenues we get, the percentage of national debt to be taken on by Scotland, the disposition of the armed forces, where Nicola Sturgeon finds the evidence to suggest we would be the sixth richest country in the OECD, whether the Bank of England will act as lender of last resort to the Scots Exchequer, and the like. But to ask these questions at all you must first concede that lots of countries smaller than Scotland are independent, and that nations have a right to self-determination. So Mr Cameron did that.

Second, it draws attention to the fact the debate about independence is not solely, or indeed chiefly, about statistics but tradition, personal attachment, heritage and human relationships. The Prime Minister's point about the number of Scots living in England, and of English people settled in Scotland, boils down to getting people to focus on whether they want to make foreigners of their in-laws or force the sister who moved to Corby to decide which nation's passport she should apply for in 2015.

Certainly, the speech was light on specifics and heavy on sentiment, but that drew attention to the fact the case for the specifics is for the Nationalists to make, and that they do not have a monopoly on national sentiment.

Mr Cameron's headline-grabbing announcement – that he would consider negotiating further devolution if the referendum on independence delivered a "no" vote – is similarly vague, of course. But it is unfair to compare it to Sir Alec Douglas-Home's position in 1979. Then, Douglas-Home was merely voicing what had been Tory party policy since Ted Heath's Declaration of Perth in 1968; it was hardly his fault that Mrs Thatcher then performed a volte-face on devolution. The difference is that Cameron actually, and actively, supports increased devolution not only for Scotland, but for all sorts of areas of public life.

Localism was, in fact, one of the big ideas of the last Tory manifesto, and it lies behind many of the party's current policies. Whether academy schools, GPs' control of their budgets, elected police commissioners, public petitions triggering Commons debates or powers for the recall of elected officials prove popular, or workable, or make it on to the statute books, they are at any rate rooted in the idea of power being exercised at the most local level possible.

The First Minister has a mandate for a referendum on independence, as Mr Cameron acknowledged. The next step is for him to present the facts which he maintains support that case. Mr Salmond does not have a mandate for adding questions about devo plus, devo max, or any other settlement. But Mr Cameron cannot be expected, and should not attempt, to lay out detailed options for this, either, until the independence question is settled. Not least because, unlike a straight "yes/no" question on separating Scotland from the rest of the UK, other parts of the country have a legitimate right to a say in any such arrangements.

The Prime Minister no doubt hopes this offer will have the effect of shooting the First Minister's fox. But that doesn't mean it isn't a genuine offer, which has the advantage of reflecting what most Scots probably want, and of being compatible with Mr Cameron's own localist doctrines.

The test of this speech's effectiveness will not be in the detail of what this further devolution would be, though. It will be whether it needs to be discussed at all.