One day when the philosopher David Hume was old and fat � scepticism didn�t stop him enjoying mutton and claret � he stumbled into a muddy bog.

Brian Morton

One day when the philosopher David Hume was old and fat - scepticism didn't stop him enjoying mutton and claret - he stumbled into a muddy bog. A passing woman recognised the atheistical rogue and offered to help him out, but only if he recited the Lord's Prayer. Hume enjoyed many more good dinners and lived on until 1776, dying just a month after the Declaration of Independence. On his death bed, he asked to be forgiven, not for the hypocrisy of invoking a God he didn't believe in, but for his Scotticisms.

There's a nice little irony here. Surely the eighteenth-century Enlightenment freed Scotland from a dub of superstition. The Enlightenment declared that good and evil were not absolute, but simply matters of perspective and adjustment, and that God was just a lazy, one-stop-shop for matters more effectively dealt with by reason and science.

It is hard to imagine the modern world without the Scottish philosophers. There would be no such thing as parliamentary democracy in British or American form; no capitalism without the economic ideas of Adam Smith; underpinning everything the discovery by geologist James Hutton that the physical world wasn't "simple and original" but a dynamic "composition" made up over time.

The Scottish Parliament sits below Salisbury Crags, where Hutton had his first intimations of this. There is a clear connection between the two structures, but one wonders how many MSPs ever consider them. It's quite possible instead that when they look up at the Crags they see them as symbolic of a louring threat. Much that was bequeathed by the Enlightenment - market capitalism, scientific reason, the sanctity of the individual - is in jeopardy. The world economy is stalled and faces the dreaded "double dip". Westminster is mired in corruption, while a Prime Minister from Adam Smith's corner of the country tries to put a bridle on the wild horses of the free market. The new US administration throws up as many troubling ironies - touching on race, religion, moral responsibility at home and overseas, economic imperatives - as it does shiny promise.

Have the great lessons of the Enlightenment, once considered to be universally applicable and for all time, simply run out of historical steam? Are we in danger - overweight and complacent as we are - of falling into the mire again? And, if so, do we need a new Enlightenment? Or are we already going through it?

The Scottish Enlightenment was not an isolated phenomenon but part of a wider intellectual movement. But while Scots philosophers and Parisian philosophes shared methods and convictions, they applied them differently and expressed them in different languages. The Enlightenment freed thought from the tyranny of Latin. Perhaps the most influential figure of the Scottish Enlightenment was moral philosopher Frances Hutcheson, who broke with tradition by giving his lectures in English. There's the rub, for Scots were already saddled with an awkward double-think, conducting their public lives in one language, their inner lives and private thoughts in another.

Hume's death-bed apology reflected a real tension for intellectuals of his generation, and one that still reverberates beyond self-loathing and social unease. Former Speaker Michael Martin was disliked as much for the way he spoke as for his failure to back reform. Our paranoia isn't entirely unjustified or masochistic. Every Scots girl or boy who has gone to an English university (which might at times include St Andrews or Edinburgh) knows that an intelligent Scot is invariably thought less intelligent than she or he is, by virtue of voice.

Language underpins everything, so perhaps what a new Scottish Enlightenment needs more than anything else is a fresh understanding of what our language is capable of. Unfortunately, because language is the medium of thought we're caught in it as thoroughly as tubby Hume was caught in the mud.

The deepest irony of the Scottish Enlightenment, in sharp contrast to those in France and Germany, is that - uniquely - there was too much light, and not enough heat. The eighteenth-century Scots philosophers may have abandoned Latin, but were still obliged to speak in a borrowed language. When Hugh MacDiarmid attempted to revive Scots as a creative language, his metaphor was volcanic, threatening to blow his top and throw out vast quantities of smoke, ash, dirt and dust in order to have his, and his country's, proper say. MacDiarmid stands at the centre of what we'd have to define as a New Darkening, not quite an anti-Enlightenment, in modern Scottish thought.

It begins in the later nineteenth century, when capitalism and empire reached their limits and started to tremor. One sees it most clearly in Robert Louis Stevenson, growing up in the Enlightenment New Town but devoted to un-reason and language as a field of conflict, not clarity. It erupts again with MacDiarmid and cools somewhat around Lewis Grassic Gibbon's time-machine approach to Scottish cultural history.

These are conventional views of the misnamed Scottish Renaissance but once again we've tended to do as we have long done with the Enlightenment: take it as a fixed and undynamic given rather than a boilingly active, evolutionary process. The Enlightenment may have tried to declare an end to "history", and our "Renaissance", with its weird mix of nationalism, internationalism, linguistic pride and linguistic subjugation, tried it on again. Truth is, we're still in that process. There could be few more telling attempts to redress the glib certainties of the Enlightenment than R D Laing's psychology - crazy is good, sane is repressive, reason is a myth - or Irvine Welsh's brilliant balancing of language registers across a fissured landscape made up of some parts solid ground and polite reason, and many steaming cracks and stinking bogs, the whole thing held together by a language whose doubleness is exactly the point. The European Enlightenment is long over. The Scots Enlightenment has been fascinatingly protracted, and if anyone asks you what its impact has been, you are entitled to say, as Chou Enlai said of the French Revolution, that it is too soon to tell.

Brian Morton is a writer, journalist and broadcaster. Anne Johnstone is away.