Early in the pandemic, Anthony Fauci, chief medical adviser to the US president, told people not to wear a mask. Later, his message changed: masks save lives. As a result, his , Fauci has lost all credibility and should go.

The immunologist¡¯s crime was to be an expert who didn¡¯t know and changed his mind. As so often has happened before, what is being attacked here is uncertainty.

The terms of argument are dictated by the critics, and they are absurdly unbalanced. On the one hand, how often do these critics ¨C?such as climate-change deniers or anti-vaccine campaigners ¨C reflect honestly on their own uncertainties? Just about never is our bet.

On the other hand, scientific uncertainty is seized on as evidence that ¡°they¡± ¨C the hated elite ¨C don¡¯t really know, are hiding something, can¡¯t be trusted ¨C and haven¡¯t we had enough of experts, anyway? Scientists¡¯ motives are doubted, their right to speak challenged (¡°if you¡¯re unsure, step aside for the real experts¡±), and their influence on policy and politics is weakened.

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The interviewer goes for the kill: ¡°So you admit you don¡¯t know?¡± Producers roll their eyes: how can they hold an audience¡¯s grasshopper-like attention with experts whose favourite word is ¡°erm¡±? As a scientist, you find yourself apologising. ¡°Admit¡± says it all: a cue for guilt and shame.

Sometimes scientists themselves accuse colleagues who express uncertainty of sowing public confusion. Even to talk of Covid unknowns was bracketed by a member of the UK¡¯s Independent Sage advisory group with ¡°Covid denial¡±, for instance.

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All this hostility to uncertainty gets under the skin, and, too often, scientists run scared. A common reaction is to treat it as a PR liability and move ever further from idealised standards of scientific self-doubt to something more like a propaganda war. The reader or listener is presented with a choice between the scientist¡¯s truth and their opponents¡¯ errors or lies.

Yet the PR approach, while tempting, is dangerous. There is always the risk of being wrong or having a change of mind, as we¡¯ve seen in history¡¯s long list of medical reversals, for example. Moreover, it also risks accusations of being untrustworthy. The ¡°climategate¡± scandal of 2009 is a prime example still quoted by climate-change deniers: hacked university emails revealed climate researchers¡¯ reluctance to disclose data to critics for fear that?they would be scoured for uncertainties. The ¡¯ verdict was?that the scientists ¡°seem so focused on winning the public-relations war that they exaggerate their?certitude ¨C and ultimately undermine their own cause¡±.

More fundamentally, uncertainty is a mark of honesty and rigour. Evidence, after all, is usually imperfect. Thus, of all science¡¯s values, the greatest is the ¡°freedom to doubt¡±, said Richard Feynman.

Caught between the forces and temptations ranged against uncertainty and its scientific inevitability, what¡¯s a scientist to do??

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Here¡¯s a suggestion. Stop ¡°admitting¡± uncertainty. Stop denying it. Insist on it.

Liberate yourself. No more running scared. Uphold uncertainty as a mark of seriousness and honesty. Advise anyone worried about fake news to put greater credence in arguments and methods that respect uncertainty.

Of course, cynical self-interest will always impersonate responsible science. who argue that even the most limited uncertainty is enough to reject the case for action ¨C as with the link between lung cancer and smoking ¨C will continue to play their game, arguing perhaps that they are just asking questions. Like drug cheats in sport, they will protest their abhorrence of cheating. But the answer to such cynicism cannot be to behave cynically yourself.

Challenged by ¡°so you admit you don¡¯t know?¡± scientists should deride the language of admission and assert that uncertainty is a norm of responsible science. They should insist that their opponents¡¯ claim to be ¡°just asking questions¡± can¡¯t be taken seriously if the questions are all on one side. They should proclaim the imperative to act on imperfect knowledge if the stakes are high, even as we keep working to reduce the uncertainty. And they should affirm the principle of operating on the basis of balancing risks, benefits and uncertainties on all sides. Vaccines, for instance, have unequivocally transformed population health, but there have been examples of adverse consequences and to dismiss consideration of them would be patriarchal ¨C and as disingenuous as anti-vaxxers¡¯ refusal to admit the benefits.

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As the television physicist Jim Al-Khalili once said: ¡°One feature that many in wider society see wrongly as a weakness is the way scientists value the importance of doubt.¡± If that is true ¨C if scientists really, truly value uncertainty ¨C then they must say so, loudly and proudly. If we must sell any message, let¡¯s sell that one.

Michael Blastland is author of The Hidden Half: How the World Conceals its Secrets (2019). George Davey Smith is director of the MRC Integrative Epidemiology Unit at the University of Bristol.

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