It is not surprising that Enlightenment thinking has become fashionable again: in uncertain times, people turn to the security promised by faith. For the signatories of the Euston Manifesto as for American neoconservatives, the cure for our contemporary ills is clear: back to the Enlightenment. For these people the Enlightenment is a holy amulet, able to ward off the evil forces of terrorism and religion while offering sanctuary to endangered liberal values.
They are half right: liberal values are certainly at risk, but it is silly to look to the Enlightenment to safeguard them. It was a hugely complex movement, and some of its most influential thinkers were enemies of liberalism. Karl Marx allowed liberal values only a transitional role in human development, while Auguste Comte, founder of the influential positivist movement, rejected ideals of toleration and equality. Yet this was not simply a battle of ideas. In the late 19th and early 20th centuries, the anti-liberal strand of Enlightenment thinking gave birth to the "scientific racism" that would be adopted by the Nazis. This ideology can be traced back to Kant's lectures on anthropology, published in 1798, in which he maintained, for instance, that Africans are inherently disposed to slavery.
As an intellectual movement, the Enlightenment has always had a distinctly seamy side. In its political incarnation, it was one of the factors that shaped modern-day terror. Right-thinking French philosophes campaigned for the prohibition of torture, but their ideas also gave birth to the Jacobin Terror that followed the French revolution. Later, Enlightenment ideas animated some of the most repressive and murderous regimes of the 20th century. Contrary to views often voiced on the left, state terror in the Soviet Union and Maoist China was not produced by national traditions of despotism. It resulted from the utopian character of communism itself. The tens of millions who starved or were killed under communism perished for the sake of an Enlightenment ideal.
What is needed today is not the return to faith beloved of Enlightenment believers and born-again Christians alike. It is realism and doubt - especially regarding the myth of progress in ethics and politics. A couple of hundred years ago, this myth may have been useful. Today, after the disasters of the 20th century, it is merely a sedative. How many times has one heard the plaintive cry "If I didn't believe in progress I couldn't get up in the morning"? The Enlightenment revival is not a return to rationality. It is fuelled by the emotions, and above all by fear.
The Enlightenment also produced some great sceptical thinkers, however. David Hume had no hopes of humanity ever being ruled by reason. A genial and tolerant soul, he would surely have been entertained by the spectacle of today's rationalists clinging frantically to an irrational faith in progress. We should follow his example, and look on the true believers in Enlightenment with a smile.