It has never been difficult for Scottish Nationalists to stigmatise unionists as less than Scottish, unpatriotic, even when not actually traitors: "A parcel of rogues in a nation." Robert Burns's scathing dismissal of the men who voted for the Union in 1707 has been quoted often, too often indeed, for it is a travesty of the truth. Many of the keenest advocates of Union then were certain it was in Scotland's interest, for "unionism," Kidd writes, "was very much a Scottish coinage...It predates not only the parliamentary Union of the Kingdoms of 1707, but also the Union of the Crowns of 1603. Deep-rooted and native, Scottish unionism was no English transplant, which partly accounts for the ways in which unionists for long happily deployed what have come to be appropriated as exclusively nationalist positions." He remarks on "Scottish assertiveness — within the Union: sometimes, of course, Scottish unionists were calling for more anglicisation than was on offer; at other times for decentralisation and greater autonomy. Above all, Scots insisted on equality within the Union" — an equality, one must add, that was also "not always on offer".

The 1603 Union of the Crowns had been fortuitous, an accident of dynastic succession. Yet it was an event which had been eagerly anticipated by the Renaissance scholar John Mair (latinised as Major) in his Historia Maioris Britanniae. "Only a union," Mair believed, "would bring about a true alignment of Scottish and English interests." He observed that "the linguistic divisions of the island did not match the political boundaries". Some subjects of the English Crown spoke Welsh, some of the Scottish Crown Erse (Gaelic), while English was the common language of lowland Britain, that is, of England and Scotland south of the Highland line. It was only around the time of Mair's death (1550) that Lowland Scots began to call their variety of the "Inglis" tongue "Scottis". The Renaissance poets, Robert Henryson and William Dunbar, wrote in what they called "Inglis" and regarded Chaucer as their master. Gawaine Douglas, Bishop of Dunkeld and translator of the Aeneid, seems to have been the first to use the word "Scottis" for his variety of the English tongue. Douglas may have spoken or understood Gaelic. Dunbar ridiculed his compatriots who wrote in "Erse". They were still to him what they had been to the medieval historian John of Fordoun, "wild Scots".

This points up one of the more curious developments of the 20th century: the Scots' willingness to regard themselves as a "Celtic" nation, something which would have astonished Lowlanders of the Age of the Enlightenment (despite their eagerness to proclaim the authenticity and antiquity of Macpherson's Ossian, presented as a translation of a Gaelic epic). When Walter Scott put Edinburgh into tartan for George IV's visit in 1822, even his admiring son-in-law and biographer, John Gibson Lockhart, criticised this "Celtification" of Scotland. Thomas Carlyle admired Robert Burns as "a piece of the right Saxon stuff". Even Lowland Scots have acquiesced in this Celtic identity. (When a professional rugby competition for Scottish, Welsh and Irish clubs was set up it was, almost inevitably, called the "Celtic League".) To be Celtic differentiates us from the Saxon English — the Sassenachs, even though this word, which is Gaelic, was for centuries applied by the inhabitants of the Scottish Highlands to the people of the Scottish Lowlands. The more homogenous British society has become, the more life in one part of the kingdom has come to resemble, and in many respects be scarcely distinguishable from, life in other parts, the more agreeable, and perhaps important, it is to mark or devise differences. To this extent the Scottish adoption of a Celtic identity may be seen as a form of unionism. It enables us to be distinct while remaining part of the whole. As the historian Tom Devine has remarked in Scotland's Empire (Penguin/Allen Lane): "The indissoluble link between tartan, the deeds of the Highland warrior, patriotism and imperial service conferred a new cultural and emotional cohesion on the Anglo-Scottish connection...The tartan cult is also a reminder that Britishness is also a part of Scottishness...Arguably, Highlandism was in large part a response to the cultural implications of union and empire."