Rule one in British politics, according to Harold Macmillan, was never to invade Afghanistan. Writing this piece I have the uneasy feeling that in British columnar journalism – although hopefully with less catastrophic consequences – rule one may be never to embark on a dispute with Neal Ascherson.

Ascherson knows more, understands more, and has written more about nationalism around the world than most of us will ever come close to equalling. He can call upon an accumulated hinterland of reading, encounters and experience that dwarfs those of us who have from time to time flirted with attempting to emulate him. Plus, he is a Scot, albeit one who basically lives in London.

Nevertheless, after reading Ascherson's summons to vote for Scottish independence this week, an argument must be joined. For Ascherson is an important intellectual luminary in Scotland's existential conversation. He speaks for and gives encouragement to many others on the pro-independence side. His speech in Glasgow a few days ago was clearly an event, described by an online admirer as "hugely significant". His text was immediately reprinted in the Sunday Herald at the weekend. And his views raise issues that resonate not just in Scotland but across Britain and beyond.

In summary, Ascherson says thatduring the course of the 20th century, Scotland went from being a partner in the union with England to becoming a dependency. The result was that London had an authority to which Scotland had to be obedient. Devolution mitigated the authority but also undermined the obedience.

Ascherson then asks why devolution is now insufficient. Two things have persuaded him of this. First, because the Iraq war showed that Britain was not an independent country. And second, because the referendum is an all-or-nothing choice, leaving independence as the only route to a better and fairer Scotland. The campaign, whatever its outcome, is in any case making Scots more independent. Its logical destination, he concludes, is an independent Scotland.

Almost every step in this argument seems to me either wrong or open to serious challenge or qualification. Did 20th-century Scotland really go from partner to dependency? Surely not. It was a partner when the union was agreed and it remains one today. Nor has Scotland become a dependency, either in a constitutional or, even more importantly, in a national or a cultural sense.

Ascherson's belief in Scotland's dependency rests on the argument that the entropy of modern history has reduced Scotland to a place to which London does unwelcome things. This is to blame the Tories for the rise of China. It is true that nations evolve internally as well as externally. Ascherson and writers such as Norman Davies have given masterly accounts of these ceaseless processes. It is also true that Britain has changed massively in Ascherson's lifetime. But these changes have transformed the whole of Britain.

Scotland has not, by and large, been singled out for mistreatment or disruption. Nor, in spite of Scotland's many real historic differences from other parts of the UK (and of the many differences within Scotland itself), has it become politically or ideologically a significantly different place from the rest of the UK since 1950 in the way Ascherson claims.

This assertion of Scottish exceptionalism, which comfortingly casts Scotland as a fundamentally more progressive, more egalitarian and more social democratic place than the rest of Britain, is an important and familiar theme of the independence debate. There are elements of truth in it. But it is often exaggerated, too often allowed to pass unchallenged. For me, it also edges far too close to the view which Ascherson, in his book Black Sea, quotes Eric Hobsbawm as always fearing more than any other, the claim that "We are different from the others – and better".

In reality, opinion polls show fewer fundamental differences of view between Scots and other British people than Ascherson and his fellow idealists allow. Just as importantly, devolution has long recognised Scotland's right to decide a large range of issues about its own evolution anyway.

Ascherson's reasons for dismissing devolution are curious. The Iraq war was certainly a disaster. Yet Tony Blair's embrace of George Bush's war was as much a snub to English, Welsh and wider European opinion, as it was to Scots. Meanwhile it is simply not true that David Cameron's refusal to put the "devo max" option on the ballot paper in September means that more devolution is off the agenda, as Ascherson implies. On the contrary, it is absolutely clear that further powers are likely to be devolved during the next UK parliament, whatever the result of the next general election.

Where Ascherson is clearly right, however, is that the independence debate has energised Britain's debate about itself. Obviously that is true in Scotland, and some of that debate has been the best revival of civic engagement in these islands for many years. But it is also happening in Wales and England, as it should, if not at the same level of intensity. Only yesterday, a Commons committee proposed new tax-raising powers for English local government. The rising chorus of English regional complaints about London is another important sign. Clearly, though, much more needs to be done.

The problems facing Scotland today — systemic problems about balancing fairness and prosperity within a globalised economy in the post-industrial, post-labour movement era — are essentially the same problems facing most developed liberal democratic economies in Europe. They will only be addressed, if they can be, by democratic reforms across national borders, not behind them. The problems are not the fault of the British state and will not be solved by breaking that state up. Scots will decide their destiny for themselves, but Ascherson's case for voting yes is more the product of his romantic radical imagination than a piece of practical and sustainable modern politics.

martin.kettle@theguardian.com