Once the men in white coats held the promise of a better future. Why h ave we lost our trust in them?

Pity the poor scientists who are deliberating over what should be done about BSE. "I almost want to crawl into a hole," one was reported as saying last week. "I look at the paper and think, my God, we've killed off a pounds 500m export industry. You can't imagine what it's like. But we have to make these decisions, and we will."

Well, I'm not so sure. I'm not so sure that it ought to be scientists alone who make these decisions; and I'm not so sure, either, that in the end the biggest victim of mad cow disease will be the British beef industry or (heaven forbid) the meat-eating public, but rather science itself. Few debates in recent years have exposed scientific expertise to the harsh light of public scrutiny quite as BSE has done, and given the way the debate seems to be going there is a real possibility that the result will be extremely damaging.

It is only relatively recently that science earned for itself a position of respect in the public domain. A little over a century ago, leading scientists like Thomas Huxley and John Tyndall fought for favour with a British establishment that was more inclined to look to clergymen than to chemists or cosmologists for "expert" judgements on the issues of the day. By 1900, science had won the ideological battle for cultural recognition; but real political influence didn't come until the experience of world war finally convinced the UK that knowledge was a crucial ingredient in industrial and military success.

Those born after the end of the Second World War grew up in a climate of extraordinary optimism about science and technology. The period from 1945 to 1965 was the heyday of deference to the scientific expert. He (it was almost always he) was the architect of astonishing new discoveries - jet-powered aircraft, atomic power, antibiotics - which were bound to make the world a better place. Hoping to cash in on science's extraordinary success, others aped its methods in supposedly scientific studies of everything from parenting (remember all those post-war childcare "experts"?) to international politics (much of mathematical game theory came out of cold-war strategic studies).

This was the time when science was generally regarded as the consumer's friend. In the early days of commercial advertising on television white- coated experts happily endorsed the latest kitchen gadgets, washing powders, toothpaste and patent medicines. In the high street, the endorsement of the laboratory scientist was an apparently automatic seal of approval, a guarantee that products were not merely new but somehow improved. If science said something was good for us, then it was good for us.

Since 1965 several things have conspired to undermine this modernist deference to science. For one thing, the instrumental success of science - itself a crucial ingredient in the rise of the scientific expert - backfired on the reputation of science, as first the disarmament movement rejected nuclear weapons and then the student movement resisted the military uses of science and technology in Vietnam.

Added to this was the growing awareness of the downside of civil R&D in the early environmental movement's protest against pesticides and pollution. In the 1950s, civil nuclear power had been a symbol of scientific and technological progress; but by the 1970s it was widely opposed by people who saw it as a symbol of all that was wrong with so-called advanced industrial society.

Today, there is a general sense that we are in transition from modernism to something not so easily described but none the less radically different. Terms such as "post-industrialism" and "post-modernism" refer in part to a less monolithic, more pluralist culture in which all of the old certainties - religious, political and scientific - are in question. Post-modern culture is altogether less deferential towards experts of all kinds: bishops scarcely count any more; politicians are widely vilified; and even scientists (the latest and in many ways still the most widely respected authority figures in our culture) have a tougher time maintaining their public reputations. In what Ulrich Beck calls "the risk society", science is no longer simply regarded as a source of solutions; it is increasingly seen as part of the problem.

The new, more sceptical attitude towards science is all around us. It is apparent, for example, in the increasing confidence with which pressure groups such as Friends of the Earth and Greenpeace contest scientific evidence on environmental issues; and it is equally evident in the increasing assertiveness of the consumer movement. Even in the courtroom, traditionally a place where scientific experts were deferred to by judges and juries alike, they have had an increasingly rough ride. In the most widely publicised trial of modern times, an American jury recently turned its back on the bulk of the forensic evidence it had been offered by electing to acquit OJ Simpson.

This, then, is the context in which British scientists are advising on what to do about BSE. Two generations ago, it might conceivably have been possible to regard BSE as a reasonably straightforward matter. A new disease of cattle having been diagnosed, the possible risks to other cattle and to humans would have been assessed by a panel of experts advisers, and appropriate action would have been put in hand by government. This, in fact, might pass as a reasonable summary of the present Government's policy. The trouble is, though, that we're living in the 1990s, not the 1950s; and a purely technocratic approach such as this is no longer creditable. Listening to current Government pronouncements about BSE is like living in a time warp; it is as if 30 years of questioning and criticism had simply not taken place.

What is wrong with simple deference to the scientific experts on BSE? First and foremost, by their own admission the scientists don't actually know very much about BSE. In fact, their ignorance of the disease - of its origins, of the nature of the infectious agent, or its mode(s) of transmission, or its host range, or its relationship with Creutzfeldt- Jakob Disease in humans - is so great that it is far from clear what solid scientific basis there can possibly have been for many of the confident and frequently unqualified pronouncements about BSE that have been issued by the Ministry of Agriculture over the past few years.

A second problem with deferring to the scientific experts in cases such as these is the problematic nature of risk assessment. In Beck's "risk society" risk assessment becomes something of a cult. Today, an almost magical aura surrounds the estimation of probable harm - despite the fact that for the most part such estimation is a mixture of science and speculation. For example, just consider for a moment what is really involved in estimating the risk to humans from the infected brains and nervous tissues of cows suffering from BSE. At one end of a spectrum of scientificity we have measurement of the levels of infectivity in different tissues, and at the other we have the daily business of the slaughtering and dismemberment of cattle. What is the scientific discipline which delivers safe verdicts concerning the reliable separation of risky from non-risky parts of cows?

Even supposing that BSE itself was better understood and that all the relevant risks were reliably calculable, it is far from clear that science alone would provide a sufficient basis for public policy. Public policy must take account of many things: the nature of BSE; the extent of the probable risks; the nature and condition of the beef industry; the state of public attitudes and public opinion; and much else besides. At best, the scientific advisers who have been in the spotlight over BSE for almost a decade are competent to judge only some of these things. Yet at times the Government has appeared to lean on these advisers so heavily that the proper boundary between scientific and political judgement has been blurred.

I should like to have answers to the following questions:

How, in a situation of enormous scientific ignorance and uncertainty, have scientists allowed their names and reputations to become firmly attached to unequivocal pronouncements by government and industry representatives on the safety of eating British beef?

Why, in a situation where science, industry and public health are all necessarily involved, have scientists alone been deemed uniquely competent to pick their way through the issues?

Why, when public confidence was always to be at least as important as public understanding and information, have scientists been left to deliberate the issues in private, without benefit even of "public interest" representatives on their expert committees?

Increasing public awareness of the true extent and limitations of scientific ignorance and uncertainty is part of our "post-modern" condition; it is part of Beck's "risk society". Ideally, the policy-making process should respond to this awareness by acknowledging the existence of ignorance and uncertainty and drawing experts, policy-makers and the public into a mature debate of the issues.

In the case of BSE, this has signally failed to happen. Instead, we have witnessed the old and dismally familiar pattern of bland political reassurance coupled with steadily declining public confidence.

In the present situation, with a major industry under threat and the extent of any public health problem still far from clear, it may seem perverse and self-seeking to worry about the fate of science. But science is important. For all its imperfections, scientific knowledge is an enormously valuable asset. In order to take advantage of this asset, however, we depend upon public confidence in science and scientists as credible sources of ideas and information in their appropriate areas of expertise. It would be a tragedy if the misuse of scientific expertise were to undermine public confidence. That way lies know-nothing fundamentalism and, ultimately, the return to barbarism.

Paradoxically, the salvation of scientific expertise in the public domain lies in a greater recognition of the proper limits of science. Our public and political cultures need a greater appreciation of what science can and cannot be called upon to do. Such an appreciation will come in part from a certain amount of well-judged modesty on the part of scientists and in part from an opening up of the processes by which scientists deliberate and decide on issues such as BSE. The days are gone when scientists could expect different rules to apply to them: if they wish their views to command public confidence and public respect, then (like everyone else) they must conduct their business openly and transparently.

Without a proper appreciation of the nature and limits of scientific expertise, the public are likely to remain caught between undue deference and undue scepticism about science. As things are at present, we seem to be moving with alarming rapidity from the one to the other.

The writer is assistant director of the Science Museum.