I believe that cows chew cud, pigs can’t fly, and night is darker than day, and also that there is water between Dover and Calais. When I believe these things, I believe them to be true. And I certainly hope that you believe them, too.

Any of us could continue the list indefinitely for ourselves. Some might put things on the list that others doubt, but there will inevitably be a great deal of overlap. Even Donald Trump has not tweeted that the USA lies north of Canada.

Scepticism about common-sense things has been on the agenda of philosophers for centuries, but only as a plaything confined to the study. It does not spill into everyday life. So, what on earth do people mean when they say we are living in a “post-truth” world?

It might suggest a worry that people are too credulous, or too quick to form attachments to outlandish beliefs. There are, after all, conspiracy theorists, fantasists, and ideologues who believe just about anything, provided that it is unlikely enough.

There is also the opposite vice of being too cautious, refusing to listen to expert opinion in areas where expert opinion has earned its authority and ought to be respected.

A person can easily exhibit both vices, being too ready to believe whatever comes across on Twitter or Facebook, and too dismissive of a consensus achieved by careful, patient, skilled investigation by trained scientists, historians or even scrupulous journalists.

Both vices tend to emerge when emotion comes into play, so that people believe what they want to believe, however flimsy or non-existent the evidence, and refuse to believe what they don’t want to believe, however well attested it is. This blight is scarcely new. It was lamented by Francis Bacon in the seventeenth century, and no doubt before that.

Perhaps our era is distinguished by a slightly different malaise. It is not so much the idea that there exists a truth about things that comes under attack, as the notion that there can be any such thing as objective inquiry into it.

So-called young-earth creationists may have dotty views about the age of the earth. But they will agree with orthodox science that there is a truth about it. They are not “post-truth,” as such. Rather, their scepticism is directed at the authority of the scientific methods of establishing what the truth actually is—a scepticism nourished by a well-protected ignorance of what those methods are and why they deserve the authority they have.

Religious conviction perverts judgement. But the inability to assess evidence properly is everywhere. In fact, the tendency to overlook or misinterpret evidence appears to be a permanent feature of human nature. Behavioural economics, for example, has found many varieties of it.

One of the most sinister is the “availability cascade,” defined as a self-reinforcing process of collective belief formation, a chain reaction in which particular stories or bits of evidence balloon into common certainties, often helped on its way by activists or “availability entrepreneurs.”

In other words, the madness of crowds, whereby a few simple memorable stories, “my child had an MMR jab and now has autism”, have greater influence than a whole library of well-conducted blind tests showing that the one has nothing to do with the other.

Once caught up in a cascade or chain reaction, people refuse to listen to evidence. They ground themselves in bubbles or silos, only listening to voices like their own. And there is no certain way of curing people who wear such blinkers.

In science, history, law, economics, or politics, the only way to recover from particular wrong turns is to go over the ground again, more carefully. Yet those who are already taken in by one particular version of the truth are unlikely to pay attention to this. As with young-earth creationists, it is the very idea of an objective inquiry that they dismiss. Applying more objective inquiry will not cure that.

Yet when somebody decries the very idea of objective inquiry, it is good to ask whether, if they were falsely accused of possessing and using counterfeit currency, they would prefer the investigation to be careful, patient, open-minded and thorough, or the reverse of all these things.

Of course, care, patience, and flexible-thinking are key features of objective inquiry. In fact, such inquiry is the only way we know about our susceptibility to error. So, we only know who possesses counterfeit banknotes when a thorough investigation shows the difference between them and real banknotes.

Unfortunately, it is easy to forget this if we talk in purely abstract terms, stoking up emotions about “Western science” or “the establishment” or “capitalism”, forgetting that under these vast umbrellas there are many different things, some of which are better established and worthy of much more respect than others.

Indeed, there is something comical about using the extraordinary results of either Western science or commercial activity—results such as iPhones, email, or even Facebook—in order to decry the very enterprises that, over centuries of hard-won economic, technological, and scientific progress, made them possible.

In any event, the growth of social media clearly facilitates cascades of misinformation. And even if these chain reactions speak to a permanent tendency in human nature, the power of Twitter and Facebook to spread untruths is part of what characterizes our objective inquiry denying a “post-truth” world.

But there are two more ideas at work in our “post-truth” era. These are not heavyweight philosophical ideas such as truth itself, or its associated tools, inquiry and objectivity. Rather, they are the moral ideas that have to do with the decline of trust and trustworthiness, and the associated idea of a loss of shame in those who parade their indifference to truth.

In a small towns and societies, it is a serious thing to be thought untrustworthy. It leads to a loss of reputation and a loss of social standing. And in the case of close-knit communities, lying and obfuscation require expiation, and a suitable and sincere expression of shame and repentance.

Alas, as society grows and become more anonymous, we get the rise of a character David Hume called a “sensible knave”: a person who seizes the advantage gained by dishonesty when they suppose they can avoid being caught out and thus avoid suffering any penalty.

Now in an age of global internet connectivity, social media offers impressionable teenagers and innumerable troll factories an unprecedented opportunity for mischief and immunity to its consequences. As a result, we begin to live in a world in which more and more people are untrustworthy more of the time.

The natural response to this is to trust people less. And in turn the natural response to that is to feel no shame when caught out, since you can readily convince yourself that many other people, perhaps all, are no better. Indeed, you can look at your social media feed and tell yourself: “That’s just politics or advertising. It is what everybody does.”

Quite possibly serial, brazen, astonishing liars such as Donald Trump or Vladimir Putin think that their reputations for honesty are no lower than those of other public figures, and this is a self-deception which it may be difficult to shift, especially in the light of the serious competition now flourishing in London.

Perhaps in the political bubble, what people say should be treated like the declamations of actors on the stage—not serious attempts to communicate truths or make promises, but mere imitations, noises demanded by the script. After all, the actor who says that he is the ghost of Hamlet’s father is not lying or even being careless with the truth. He is in a different game altogether.

This would be a reassuring interpretation of what goes on in London, Washington, or Moscow. But in the theatre when the lights go out and the actors and audience go home, the make-believe gives way to the serious business of living, and truth and reason regain their sovereignty.

Unfortunately, it doesn’t work like this in politics. Untruths and “acting” spill out of the political theatre, infecting everything. It is only fun and games until things stop working, the food banks multiply, and people start marching, or dying.

So, what is the cure? The only remedy for bad ideas and bad mental habits is the cultivation of better ones. We need leaders to set better examples and we need to raise people good at distinguishing what is trustworthy from what is not.

Clearly, this is not going to be achieved by a Gove-like instilling of facts, or formulae, or grammar, which merely trains children in the bovine receptivity that is the very opposite of any active, intelligent, and critical response to the world.

What we need is an education system that encourages cautious scepticism and an imaginative open-mindedness, allied with the sensible assessments of probabilities. We also need to develop dispositions towards decency and civil debate.

In a “post-truth” world characterised by cascades of misinformation and politicians with no shame, we ought to bring the practices of philosophy into our classrooms.

What a subversive thought! But then as the saying goes, if you think knowledge is expensive, try ignorance.

Simon Blackburn is a fellow at Trinity College, Cambridge. He is the author of Truth: A Guide for the Perplexed and Think: A Compelling Introduction to Philosophy.

This article is part of the Agora series, a collaboration between the New Statesman and Aaron James Wendland. Aaron is assistant professor of philosophy at the Higher School of Economics and the co-editor of Wittgenstein and Heidegger and Heidegger on Technology. Follow him on Twitter: @ajwendland.