A great American poet, John Ashbery, wrote that tomorrow is easy, but today is uncharted. He put his finger on our real weakness. It is not our ignorance of the future, which is incurable. It is our failure to understand the present.

Our view of the present time is overlaid with after-images from the recent past. At the moment, our idea of what it is to be modern is one of those after-images. We have inherited the faith that as the world becomes more modern it will become more reasonable, more enlightened and more balanced. We expect that, as modern habits of thinking advance across the world, people everywhere will become more like ourselves - or at least as we imagine ourselves to be.

We think a truly modern world will be forward-looking and secular - not perhaps irreligious but certainly not fundamentalist - and that its values will be humane and cosmopolitan. We remain confident that the growth of scientific knowledge will allow us to control the risks of life and eliminate its worst evils - war, hunger and sickness.

Not all of this optimism is illusory. De Quincey's remark that a quarter of human suffering is toothache is worth remembering if ever we're tempted to think that little in the human lot can be improved. The contribution of anaesthetic dentistry to human well-being is a reminder that, in some fields, there have been unalloyed improvements, real progress. Even so, faith in progress is a superstition.

Nineteenth-century thinkers such as Karl Marx and John Stuart Mill believed that, as society and the economy become increasingly based on science, so our outlook on the world would become ever more rational. This is not to say that either man believed progress to be strictly inevitable; they did not rule out setbacks to the progress of reason. But what they never envisaged was that irrationality would continue to flourish alongside rapid advances in science and technology. The chief tenet of the Enlightenment is that the growth of knowledge is the key to human emancipation. No true believer in the Enlightenment would ever question that article of faith. Yet, at the close of the 20th century, faith in progress through the growth of knowledge itself looks irrational.

In fact, there is no consistent, enduring link between the adoption of modern science and technology on the one hand and the progress of reason in human affairs on the other. If anything, new technologies can give a new lease of life to the side of human nature that is not and will never be rational. The Taliban commander directing military operations from his cellular telephone is a familiar late 20th-century figure. Neither the modernisation of the economy nor the spread of new technologies leads to the adoption of what we like to think of as a modern, rational worldview. Just as often, the result is fundamentalism.

This isn't a development confined to poor countries. In the United States, which sees itself as being at the cutting edge of modernity, political life is riven by religious conflicts. Neither America's continuous civil war over abortion nor the attempt to drive Bill Clinton from office can be understood unless we grasp that the world's most modern country is also the one in which a fundamentalist minority can shape the political agenda. In Britain, the cult that surrounded the death of the Princess of Wales suggests that, while this country is certainly post-Christian, it has not become secular. Traditional religion is in retreat but it has not been replaced by rationality. Modern societies are full of occult and millenarian cults. They abound in new, short-lived religions, "flickering and fading", as J G Ballard has put it, "like off-peak commercials".

With the 19th-century faith in progress we have inherited the belief that we can control the technologies we have invented and use them to advance human well-being. But think of the genetic technologies that are altering the food we eat and which make the cloning of human beings a realistic possibility. Some people see in these technologies the prospect of a great leap forward in which we shall defeat hunger and disease and eradicate disability. Others - and I'm among these sceptics - welcome the potential benefits but fear that our new powers to remake nature and ourselves will be used in the service of hubristic fantasies. We can't renounce technology, and the idea that we can is just hubris in another guise. But we should recognise that, if we redesign nature to fit human wishes, we risk making it a mirror of our limitations. A world that has been rebuilt into a factory producing the things humans need or want will be a world without wilderness and from which many of the species with which we share the planet will disappear.

Who wants to live in such a world? What will human beings be like when they have been genetically re-engineered? And what will racists and tyrants do with the new power to remix human genes? If history is any guide, a world remade by genetic engineering may have a closer resemblance to H G Wells's Island of Dr Moreau than to the dreams of today's technological utopians.

There is no power in the world that can ensure that technology is used only for benign purposes. Partly this is because we cannot agree on what such purposes are. Partly it is because even when enough people are agreed there is no power that can enforce the consensus. The institution on which we would have to rely for such enforcement - the modern state - is not up to the job.

In large parts of Africa, in much of Russia and some of its neighbours, in Colombia, Afghanistan, Pakistan and many other countries, the state has collapsed. In so far as anyone still has power in such countries, it has passed into the hands of tribal or religious militias, shadowy and fissiparous political organisations, criminal cartels or clans. The world today isn't a single political system any more than it is really a single economic system. It contains perhaps three or four dozen genuinely modern states with effective governments. Many other states are enfeebled or highly unstable; in large parts of the world, government has broken down and life has reverted to a pre-modern, semi-anarchic condition.

And economically the world is ruled by a sort of ramshackle laissez-faire. In a worldwide free market, corporations that try to exploit the commercial potential of genetically engineered crops will not be stopped by prohibition or tight regulation in one country or one group of countries such as the European Union. They will relocate to countries where such restraint is slight or non-existent. That's not difficult when there is such a large choice of sovereign jurisdictions in the world and capital and production enjoy unfettered mobility between them.

The history of international efforts to stem the proliferation of weapons of mass destruction - nuclear, biological or whatever - offers little support for the hope that the spread of new technologies can be controlled. It is worth pondering how, even when the world's most powerful governments have an urgent and enduring interest in containing the spread of such weapons, their best efforts have succeeded only in slowing it down. It's hard enough to enforce nuclear non-proliferation agreements on a hundred or so fractious states. It's even harder to make them effective when, in many parts of the world, governments are no longer in charge.

Once a new technology is out in the world anyone can use it. At that point it becomes a weapon in human conflicts and an embodiment of human dreams. We are not masters of the tools we have invented. They affect our lives in ways we cannot control - and often do not understand. The world today is a vast, unsupervised laboratory, in which a multitude of experiments are simultaneously under way.

Many of these experiments are not recognised as such. But the industrialisation of agriculture, and the intensive use of fertilisers, pesticides and antibiotics to render plant and animal life unnaturally productive; the use and over-use of antibiotics in medicine; the widespread use of chemicals, illegal or prescribed, to alter mood or behaviour; mass air travel on the scale of the past 20 years or so - these and many other practices, which we have come to take for granted, are really experiments, carried out on a scale that defeats our understanding.

We can't know the risks of these experiments because we don't understand their interactions with one another. We can't control our new technologies because we don't really grasp the totality of their effects. And there's a deeper reason why we are not masters of our technologies. They embody dreams of which we are not conscious and hopes that we cannot bear to give up. Late modern cultures are haunted by the dream that new technologies will conjure away the immemorial evils of human life.

But no new technology can abolish scarcity, do away with the necessity of choice or alter the fact of human mortality. For generations, techno-utopians have been telling us that tyranny would soon be obsolete. It is not so long since we were assured that television would make war impossible. Today there are those who believe that the virtual communities of the Internet will make dictatorship unworkable. There are even people who imagine that freezing our bodies or brains soon after the moment of our death could make us immortal.

Such techno-utopian fantasies deny some very obvious truths. New technologies strengthen repressive governments as well as weaken them. The rulers of China did not try to hush up the massacre in Tiananmen Square. They filmed it on video cameras and used it as a warning to others.

Equally, new technologies, even as they make it easier for people to communicate with one another, make it harder to achieve or protect privacy. We've entered an age in which anonymity has become virtually impossible. And, by themselves, the virtual communities of the Internet are powerless to overthrow tyranny. For that, the long haul of political struggle, with all its setbacks and sacrifices, is needed.

The idea that technology can do away with death encapsulates the most absurd denial of facts. Let's make the very large concession that preserving human bodies in a condition that allows them to be resuscitated is technically feasible. Even so, no technology that requires human bodies to be stored for generations or centuries is proof against war, changes in regime and the recurrent breakdowns of law and order that have occurred in every society of which we have knowledge.

When we look to technology to deliver us from accidents, from choice and even from mortality we are asking from it something it cannot give: a deliverance from the conditions that make us human. We are seeking from technology what we once looked for in political ideologies, and before that in religion: salvation from ourselves.

The dream that technology serves for us is a dream of complete control. It's a dream with ancient sources in western traditions. It's the dream that we can cease to be mortal, earth-bound creatures subject to fate and chance. It's a product not so much of science as of magic. The project of using technology to remake the world according to our will captures the fantasy by which we have been ruled during much of the 20th century. It was a fantasy of progress without instability.

The writer is professor of European thought at the London School of Economics. This essay is based on a recent Radio 4 talk