We might dislike it, but we must take a lot of what we learn on trust - just keep a pinch of salt handy, says Simon Blackburn

Whatever else Tony Blair's cherished legacy may be, few predict that it is going to include an increased trust in government. But these days our mistrust includes more than government. We do not trust the press. We do not trust drug companies, doctors, lawyers, accountants or surveyors, let alone employers and vice-chancellors. We are ourselves not trusted.

And we retaliate by mistrusting those who would manage us, especially their competence, league tables and performance indicators. We probably do not trust our colleagues very far. It is easier than ever to fake things, and when it is easy it is going to be done. So the attitude elegantly expressed by lawyers with the question " Cui bono ?" (to whom the advantage?) - and less elegantly asked by astringent political interviewers in the form: "Why is this lying bastard lying to me?" - guides just about every human interaction.

Except that it cannot. As philosophers have repeatedly pointed out, we have no option but to take things on trust. If you could cleanse your mind of things that are there only because of the word of others, you would end up with a very blank mind indeed. Let us waive the problem that language itself might have to be erased because you took a lot on trust as you learnt the names of things and the meaning of words. Even with language in place, each person's experience is fragmentary, confined in space and time, and uninterpretable in any interesting way without the tools forged over innumerable centuries by other people.

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Subjects who know things never work them out off their own individual bats. They have been told things - which they trusted and which they can bring to bear, often without thinking about it. "What you have inherited from your forefathers you must first win for yourself, if you are to possess it,"intoned Goethe, but the implied ideal of self-sufficient individual experience is a chimera. If those people who are as clever and persistent as Copernicus are the only ones who can rationally believe that the Earth moves round the Sun, then not many are going to qualify. And, in any event, Copernicus stood on the shoulders of others as well.

It is usually said that the glory of science is its self-correcting nature. Observations and experiments can be repeated, and a mistake or false claim will be detected as others fail to replicate a reported result. This is often true, or so I believe. But mostly I believe it because I have read reports of it happening and believed them.

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We glimpse here the spiral down into scepticism. "It is all very well," says the conspiracy theorist, " but cui bono ? To whose advantage is it that we believe that science is self-correcting?" And thus the scientific expert is met by the same raised eyebrow and sly tap on the side of the nose as the salesman. This excessive scepticism is, of course, unwarranted, but not because of Goethe's standard. I cannot myself go out and replicate attempts to produce cold fusion in the laboratory, for instance, because I would not know how to go about it. And if a physicist tried to teach me, I would need to take nearly everything he said on trust. It is not just that I am unfortunately not a scientist. Scientists will be in exactly the same boat when they stray a step from their particular area of expertise, the one into which they were educated by trusting their teachers.

So we seem to be caught in a bind. Truth will no doubt prevail, but it will take others to tell us what it says. So, however much we feel we cannot trust others, we have to. The way out must be to proceed piecemeal, feeling our way to trusting only those people on only those subjects on which they are trustworthy. We edge forwards, extending our sense of what is probable, one piece at a time. We have to practise and, if we are lucky, we develop the sifting habit and a nose for snake oil. We learn to discriminate.

The tabloid newspaper, which is completely untrustworthy on the economic impact of immigrants, reports football results flawlessly. Because of this, the lawyer's tag " falsus in uno, falsus in omnibus " is itself untrustworthy. Very few of us go under false names, for example, but not many can be trusted to tell their doctor how much they drink.

It takes time to confirm that anyone is untrustworthy across the board. It took some people seven years to reach that conclusion about new Labour.

Fortunately, we have other rules to guide us, as well as pieces of folk wisdom such as never trusting anyone who starts a sentence with "Trust me...". If someone has a lot at stake in your believing something that seems extremely unlikely, then be on your guard, more so than when someone has nothing at stake in your believing something quite probable. "The wise," said Hume, "lend a very academic faith to any report that flatters the passions of the reporter" - and I trust him on that. It is equally true when we are ourselves both reporter and audience. If we find ourselves relishing the shocking details, then we have to beware whether we were more than usually gullible when we accepted them.

A Bayesian account of all this has us balancing the antecedent likelihood of what is reported; the likelihood of whatever event was reported being reported just like that; and the antecedent likelihood of a report such as that coming along anyway. The first is what sinks reports of miracles; the second is what makes us suspicious of Ishmael's allegedly first-person report of the events in Moby-Dick ; while the third was immortally caught by Mandy Rice-Davies's reaction to some aristocrat's denial that he had had sex with her: "Well, he would say that, wouldn't he?"

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There are two broad families of ways that people become untrustworthy. First, there are straightforward failures of honesty, as in lies and fraud. Second, and more interesting, there are failures of the reality principle or the brakes that usually prevent people from believing what they like, or what they wish were true, or what it is convenient or pleasant to believe. The problems here may be put under the heading of fantasy, of living in a dream. The two can blur into one another, and historians of our age will have a hard time disentangling them.

Some professions, such as public relations and the clergy, depend wholly on the failure of the reality principle. But it is a mistake to suppose that such people are thereby shown to be blameless for inflicting their fantasies on others. They may be negligent or reckless in their concern for the truth, without deliberately setting out to deceive. However, although it is philosophically hard to understand, the active voice of the verb "deceive" shows that people suppose that when we deceive ourselves we actively comply with our escape from reality. We are not passive victims of a piece of bad luck, but agents of our downfall.

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Just as we need to edge towards trusting a source, so we need practice to become trustworthy ourselves. In situations of total hostility, there can be no such practice. Often the first test of trust would be, as Hobbes put it, "betray yourself to your enemy", and, rather than risk that, out come the weapons again.

Philosophers tend to agree that when practice is possible and the process works, something rather magical comes about, a kind of transformation in which the question cui bono ? becomes inapplicable. It becomes out of the question that our own advantage drives our saying what we do, in competition with the truth of what we are saying. Our loyalty to the truth becomes immune to the temptations of gain or the fear of loss.

Similarly, when we give a promise, the question of performance, ideally, becomes immune to the calculations that might have affected our decision-making had we not made that promise. It is not that countervailing temptations (any advantages of misleading the audience) are weighed and found insufficient to offset the advantage of keeping the promise. Rather, any such temptations are "silenced", deprived of any vote in how we behave. They are not even present as temptations: the question of defecting from those standards simply does not arise, any more than the question of whether to cover the pretty stranger sitting opposite us on the Tube with burning kisses. The idea may cross our minds, since we are but human, but it will not be felt as a temptation - but people break promises all the time.

Economists, especially, trained to look on rational behaviour as informed by nothing except a calculus of self-advantage, find it difficult to believe that integrity can take on a life of its own. Not believing it of others, they will find it harder to practise it themselves. This is confirmed by the evidence that in repeated games such as the famous prisoners' dilemma - in which two players do better if they co-operate than if they each defect, but even better if one defects while the other co-operates - the groups that do worst tend to be economics students. Unbridled pursuit of self-interest may sometimes reward the collective with an invisible hand, but in these cases it ruins it with an invisible boot.

When our integrity has taken on a life of its own, we become angry if others insist on raising the question of whether our sayings are serving our own advantage. And once we are prissy in these respects, we find it embarrassing to raise the question about other people who may be soliciting our trust. This is why we are taken in again and again by the broad smile, firm handshake and ringing tones of the con man. I find it hard, however, to believe that things are worse now than they ever were.

Even economists have some authorities on their side. Consider Machiavelli's advice to the Prince, to appear to have virtues, including trustworthiness, without actually having them. According to Machiavelli, "therefore it is unnecessary for a prince to have all the good qualities I have enumerated, but it is very necessary to appear to have them. And I shall dare to say this also, that to have them and always to observe them is injurious, and that to appear to have them is useful; to appear merciful, faithful, humane, religious, upright, and to be so, but with a mind so framed that should you require not to be so, you may be able and know how to change to the opposite." Plato recognises the insidious allure of such advice and attempts to rebut it in The Republic. Machiavelli, however, illustrates its applicability with plentiful examples from his own time. We should not be surprised if ours furnishes him with more.

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Simon Blackburn is professor of philosophy, Cambridge University.