Put the two together and you have a worrying prognosis. The clash of civilisations is happening not between Islam and the west, as we are often led to believe, but between pragmatic relativism and dogmatic certainty. On this analysis, it is easy to see liberal democracy not as the crowning achievement of civilisation but a manifestation of a laissez-faire, morally bankrupt modernity. "Relativism appears to be the philosophical foundation of democracy," said Ratzinger in 1996. "Democracy in fact is supposedly built on the basis that no one can presume to know the true way."

It is no surprise that both the MoD and the Pope believe that the beneficiaries of this polarisation will be those offering certitude, since belief in something is almost always preferable to belief in nothing. As Walter put it in the film The Big Lebowski: "Say what you like about the tenets of national socialism, Dude, at least it's an ethos."

How did we get to this dismal Hobson's choice? The finger of blame has to be pointed largely at academics and intellectuals who have been so keen to debunk popular notions of truth that they have created a culture in which the middle ground between shoulder-shrugging relativism and dogmatic fundamentalism has been vacated.

Of course, the works of truth-deniers such as Michel Foucault and Richard Rorty are hardly bestsellers. Yet their ideas do filter through to society as a whole. Consider, for instance, how what passes for common sense about morality has been turned on its head. For millennia, most people believed that right was right and wrong was wrong, and that was all there was to it. Now, university lecturers report that their fresh-faced new students take it as obvious that there is no such thing as "the truth" and that morality is relative. In educated circles at least, only the naive believe in objectivity. What was shocking when Nietzsche first proclaimed it at the end of the 19th century became platitudinous by the start of the 21st.

Perhaps the most powerful idea to filter through from the universities to the streets was articulated by Foucault, who adapted and popularised the Nietzschean idea that what passes for truth is actually no more than power. There are no facts, only attempts to impose your view on the world by fixing it as "The Truth". This idea is now so mainstream that even a conservative like Donald Rumsfeld could complain about those who lived in the "reality-based community", arguing "that's not the way the world really works anymore ... when we act, we create our own reality."

Most Anglophone philosophers find this kind of hyper-scepticism absurd and pernicious. But although these ideas were hatched by philosophers, they have gained wide currency in the humanities and the social sciences, often in bastardised form.

Some philosophers, such as Bernard Williams and Simon Blackburn, have waded into the public debate in an attempt to put the relativist genie back into the bottle. Books such as Why Truth Matters, by my colleagues Jeremy Stangroom and Ophelia Benson, have also tried to stem the tide. But this is not really a highbrow academic debate about whether there is Truth with a capital T - it is about how abstract ideas relate to the business of everyday life.

Richard Rorty, for example, argues against Truth brilliantly, and it is far from clear that he is simply wrong. The problem is that he does not concede as unequivocally as he should that in practice his theories usually leave the world more or less as it is. Rorty believes as much as anyone else that the Holocaust happened more or less as described in history books, he just refuses to use an allegedly outmoded vocabulary of truth to say so. It is not quite fair to call his refusal in such contexts a pose, but it is certainly not quite what it seems.

Ironically, like many left-leaning intellectuals, Rorty thinks that denying objectivity and truth is politically important, as a way of liberating people from the ways of seeing the world promoted as the Truth by the powerful. However, it turns out that Rorty and his ilk seriously misjudged what happens if intellectuals deny truth stridently and frequently enough. Far from making liberal openness more attractive, such denials actually make it appear empty, repugnant and weak compared to the crystalline clarity and certainty of dogma.

They owe us an apology for failing to either see themselves, or make it clear to others, that in the everyday world we can and must distinguish truth and falsity, right and wrong, even if on close examination these terms do not mean what we thought they did. Science may not be God-like in its objectivity, but it is not just another myth. Moral values must be questioned, but if discrimination against women, homosexuals or ethnic minorities is wrong here, then it is wrong anywhere else in the world. Truth may not be the simple phenomenon we assume it to be, but falsehoods must be challenged.

Unless we can make a convincing case that the choice is not between relativism or dogmatism, more and more people will reject the former and embrace the latter. When they do, those who helped create the impression that modern, secular rationality leaves everything up for grabs in the marketplace of belief will have to take their share of the blame.

· Julian Baggini is the editor of the Philosophers' Magazine and author of Welcome to Everytown

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