Relativism and scepticism are strange bedfellows. In the public mind they are not sharply separated, I suspect, since the relativist, like the sceptic, is supposed to suspend judgement in places where other, more red-blooded people, like Hilaire Belloc's dons "Who shout and bang and roar and bawl/ The absolute across the hall", want belief and conviction. But in reality they are diametrically opposed. According to the relativist, belief and conviction fly out of the window because truth is, as it were, too cheap to care about. There is too much of it about: your truth, his truth, and my truth. For the sceptic, belief and conviction fly out of the window because truth is too rare. We cannot care about it because we cannot find it; we cannot even search for it because we cannot tell when we are getting closer. Unlike the relativistic frame of mind, that of the sceptic is often admirable. The relativist reflection is dehumanising. Its attitude, including its light irony, is the stance of someone above the fray, someone who has seen through the debates and engagements of ordinary participants. But this stance is demeaning and impoverished, a mere distraction from whatever issue concerns us. By contrast, the sceptic makes no attempt to bypass or sideline the issue. The issue is the issue, and so is truth. It is just that according to the sceptic, we cannot find the truth. We must moderate our opinions, confess to our ignorance, avoid conviction and dogma because we recognise the inadequacies of our investigations or our methods.

The British, fortunately, have strong sceptical leanings, which is partly why the roaring and bawling of the present government is so despised. Americans, by contrast, have a natural appetite for belief. According to one account I have read, it is not only that around 90% believe in the literal truth of Christianity, but 49% believe that people are sometimes possessed by devils, and three-and-a-half million believe themselves to have been abducted, at some time or another, by aliens. This is disturbing, for none of those beliefs are guaranteed to remain inert, especially when times are fearful: we may remember those unfortunates a year or two ago, who believed that the Hale Bopp comet was a spiritual recycling facility for dead Californians, and killed themselves so as to go to join it. As Voltaire said, those who can make you believe absurdities can make you commit atrocities.

Nevertheless, scepticism has its limitations, and I would like to explore one of them. Let me start with a complaint. In 2001 the great American philosopher Willard van Orman Quine died. His death prompted an ignorant and shameful article in the Times by their columnist Simon Jenkins, lamenting that Quine was the kind of philosopher who lived and wrote remote from the everyday: a typical example of the useless intellectual. And indeed, unlike Belloc's dons, Quine trumpeted few absolutes. Yet neither was he a sceptic. So what good did he do?

Well, Quine was probably the most important theorist of knowledge of the latter half of the twentieth century. He gave a subtle, original, and comprehensive theory of the proper process whereby experience should be transmuted into theory. Quine knew that none of the avenues to knowledge is simple, or infallible, or immune to endless revision and question. Neither the senses, nor testimony, nor history nor theory nor reason itself, gives us bedrock. In his favourite metaphor, borrowed from the positivist Otto Neurath, we are like sailors condemned to rebuild our boats at sea. No part is immune to critical inspection, and each part can be replaced, but we must stand on other parts as we do it. The only rational process is to discover what works, and to warp our scientific heritage as cautiously as possible to cope with the recalcitrant experiences nature brings our way. This is the way of science, with its virtues of observation and experiment, of conjecture and refutation, of open debate. Science should be seens as a Darwinian process whereby a plurality of theories compete for credibility, and only the fittest survive, perhaps only for one lifetime, in the endless process of self-correction.

In saying these things Quine was partly echoing the American pragmatist CS Peirce, famous for the much-criticized definition of truth as the opinion which the progress of science is fated to converge on in the long run. But the long run is only an imaginary focus: the process is itself guaranteed to yield improvement at each step on the way. It is because he believed in this process that Quine was not a sceptic.

There is, however a place where a different set of processes enter. Peirce and Quine are perhaps apt to describe science as a kind of self-enclosed enterprise, driven by an inner logic, and needing no support from its surroundings. But it is of the utmost importance to see that this is false, and false in many dimensions. It is most obviously false because institutional science needs support. It needs the leisure of inquiry, which in turn needs investment. It is false too because the whole Darwinian process only works given virtues of integrity, communication, toleration, and open-mindedness. Science could only flourish when religion lost the power to stifle those virtues, and it still cannot flourish where religion or other forces retain that power. Science, in other words, needs an entire cultural and political matrix in which to grow properly, and nothing in that matrix can be taken for granted.

We see small examples of this in particular parts of the scientific enterprise, perhaps most notably medicine. The wise Hume told us, lend a very academic faith to any report that flatters the passions of the reporter. Few of us are taken in when the American Psychiatric Association voted to medicalise naughtiness, inventing instead attention deficit disorder, and so opening the way for one in seven children in the country to be regularly and profitably prescribed Ritalin, a class-A drug with sedative effects. When the government here refused a foot-and-mouth inquiry, they instead appointed old friends and colleagues to report on the virtuous conduct of old friends and colleagues, and Hume's dictum predicts the way such an 'inquiry' will be received. More insidious cases of mass hallucination probably depend in the first instance on institutional needs of particular sciences. So, for instance, the United Nations Intergovernmental Panel on Climate Change has relentlessly issued graphs and reports testifying to the imminent and catastrophic effects of global warming. For the scientists on the panel such claims justify further funding, not to mention increased institutional power, computer time, and first-class air travel to exotic conferences. The passion that makes us receive these reports, like those of other environmental disasters, so avidly is, I suppose, that of guilt. For there is in fact only the poorest evidence of any atmospheric warming, and excellent evidence that there is either none or next to none, just as there is no evidence of rising sea levels and none of increased climate violence (the poor evidence for derives from arbitrarily scattered surface measurements; the stronger evidence against derives from satellite data that covers virtually the entire globe, and meteorological balloons).

My purpose here is not to qualify my message by belittling science. On the contrary, the data are the result of painstaking, excellent, impeccable science. But a public statement can sit on the top of a mountain of science without adequately reflecting that science. It is the public statements, whether of the American Psychiatric Association, the government, or the Intergovernmental Panel on Climate Change that need to be taken with a generous pinch of salt. Hume also quotes with approval a saying of La Rochefoucault that there are many things in which the world wishes to be deceived. The religious impulse is one manifestation of this truth. But another is the impact of emotions, including fear and guilt, upon belief, and this is the mechanism that leads us to receive messages of doom and disaster with our critical faculties asleep. And this brings us back to a question close to that of relativism.

The West, it is sadly said, has lost confidence in the Enlightenment. It is quite common to see intellectuals state as a fact that the Enlightenment project has been tried and failed. This is a lie. There never was one single Enlightenment project, and of the Enlightenment projects that there were, many have succeeded beyond the wildest hopes of their proponents. The Enlightenment provided the matrix I have talked of, in which scientific enterprises could flourish. Now, our understanding of the world is better because of physical science. Our understanding of ourselves is better because of biological science. We live longer, and we feed ourselves better, and "we" here includes not only people in first world countries, but countless people in the third world. We look after the environment better, and in time we will manage our own numbers better. Outside the theocracies of the East more people have more freedoms and enjoy more education, more opportunities, and may even have more rights than ever before. We owe this progress entirely to the culture forged, in the West, by Bacon and Locke, Hume and Voltaire, Newton and Darwin. Humanism is the belief that humanity need not be ashamed of itself, and these are its great examples. They show us that we need not regard knowledge as impious, or ignorance as desirable, and we need not see blind faith as anything other than blind.

[This is an edited extract from the Voltaire Lecture 2002 organised by the British Humanist Association]