The surface of the world rises and falls, from the depths of the oceans to the summit of Mount Everest, in a single continuous line that we can follow without lifting our finger from the map. Between the darkest depths and the highest summit there is but a difference in height, and we can pass from one to the other. However, the simple act of reading two different newspapers or listening to a conversation between intellectuals coming from different fields of knowledge reveal a different kind of chasm, a fractured and discontinuous landscape where the distance between A and B is infinite because going from A to B is simply impossible.

Suddenly, facts are facts to the extent that they fit the desires of each group, of each tribe. Each of them develops its own language, one that, of the many functions of language, favors the ability to trigger emotions, and makes those emotions build landscapes only accessible to those who see them in the same light. As the world is one but views are diverse, tribal discourse gradually separates and polarizes. In The Death of Tragedy, 1961, George Steiner said: "Words carry us forward toward ideological confrontations from which there is no retreat. (...) Slogans, clichés, rhetorical abstractions, false antitheses come to possess the mind (...). Political conduct is no longer spontaneous or responsive to reality. It freezes around a core of dead rhetoric. (...) Instead of becoming masters of language, we become its servants.”

And in the wasteland surrounding the abyss, in the territory that each tribe calls its own, grows a harmful seed: post-truth. 

Annually, the Oxford Dictionary chooses the "word of the year". In 2016, that word was post-truth, defined as "circumstances in which objective facts are less influential in shaping public opinion than appeals to emotion and personal belief." 

The most frequent use of the term post-truth is in politics. The politics of post-truth was widely discussed in the context of Brexit and the 2016 U.S. presidential election. In both cases, the winners relied on facts that turned out to be clearly false (that Brexit made sense economically) or on vague slogans ("make America great again"). There were exaggerations, instances of misinformation and false promises, as if politicians had decided to illustrate H. L. Mencken's phrase: "There is always a well-known solution to every human problem —neat, plausible, and wrong". In both cases, an atmosphere of exacerbated polarization was generated: what we say is right and what they say is wrong, regardless of whether it is true or not. 

Lies are not new to politics. Thucydides already mentions them in his writings about the Peloponnesian War (5th century B.C.E.), when he says: "In order to adapt to all the changes and events, words also had to alter their usual meanings." What is novel is that, when it became clear that the Brexit and Trump campaigns were awash in false data, many voters did not feel cheated. As if truth itself, in its broadest or even narrowest sense, was no longer relevant to them in the face of how they were made to feel. Some politicians even gave up trying to appear truthful, making claims that could easily be disproven by anyone. 

Besides politics, post-truth is often discussed in the field of journalism and professional communication. Traditional media are being displaced by new media. Social networks have made it easier than ever before to share news, both true and false. We can all post new content that quickly adds to and mixes with what is already available. In a few minutes, news of an attack or an earthquake can travel around the world, but so can a rumor, fake news or mundane gossip. On the one hand, the ability to generate and consume content in parallel to traditional media gives us independence and freedom. On the other hand, it sometimes becomes especially difficult to know how to value each particular piece of information. Our separation into tribes, each with its own "pseudo-reality", is strengthened by the way we connect to the media and use social networks, which make it easier for us to gather in echo chambers.Post-truth therefore jeopardizes, above all, the possibility of generating and maintaining human bonds that are only made possible by living together in a shared world.

Some believe that we should not speak of post-truth but simply of a lie or falsehood. But in the post-truth era, facts are hidden, shaped and manipulated, sometimes deliberately and systematically, sometimes not. 

Apparent certainties arise where there are still doubts, and also apparent doubts where there are already certainties. This confusion creates post-truth, which appears to be a cohesive and systematic narrative in which internal coherence matters more than a base in reality. This is neither a mistake nor a lie: a mistake can eventually be corrected, and liars know that they are lying. On the other hand, it is neither easy to detect post-truth nor to get out of it, because it equalizes everything. 

It is too soon to tell if we are in a "post-truth era," but we do know that this phenomenon can be observed in areas beyond politics or journalism, areas where we also have data, we know things, and yet some of us brush all that aside and adopt a position that is not supported by the facts. We know that vaccines are mostly safe and very effective in preventing disease, and that humans are largely responsible for the global warming that threatens our survival. However, some believe that vaccines cause autism, or that anthropogenic climate change is a lie. These examples, in which unreasonable doubts creep in (what we know about these topics we know very well), reveal a structure of post-truth that we also see in politics or journalism.

Discussing post-truth forces us to discuss truth, a word that means different things depending on the context. The concept of truth is very precise in mathematics, logic or metaphysics, where truth is obtained deductively. That is not the truth that we will discuss. We will instead define truth as the correspondence between what we say and what happens in the world. Our approach will be rather practical. That is, we will assume that there is a reality, independent of us, and that we can access it with a greater or lesser degree of difficulty. Our access to reality is imperfect because it is through imperfect tools: our experience is subjective, our senses mixed with our expectations tell us what is going on, and our interpretations of what the facts mean may vary. We could cry over spilt milk, complain about our limitations, or accept that this is the best we can do and consider those limitations as part and parcel of our access to reality. For our purposes, truth should neither be considered absolute and totally certain nor so vague and inaccessible that it borders fiction. And this delicate distinction is one of the central points that we will discuss in the chapters that follow. 

So, there is a real world out there that seems to have its own rules and in which things happen. Those things that happen are facts, real facts. There are no "alternative facts". 

Having made this clear, let's return to post-truth. Highlighting emotional aspects and ignoring the information you have so that you can take a contradictory position is not always intentional. Sometimes, and perhaps this is one of the central problems, what we see is a certain indifference in the face of the very distinction between what is false and what is true. Emerging from this disinterest or inattention on our part is what we will call unintentional post-truth. Many factors play a part, such as our beliefs or emotions, the fact that we fail to distinguish real experts from false experts or that we tend to make mistakes in our reasoning. We are separating into increasingly isolated tribes, each one gathering around common ideas that are often far from what is real, from the truth. Moreover, the media increasingly amplify the most extreme voices because that makes them more reliable in the eyes of an audience that, after all, expects them to say exactly what they want to hear. 

When these unintentional post-truth-generating mechanisms are adopted and exploited by groups interested in promoting a "post-truth" narrative, we have intentional post-truth. But this post-truth could not occur if it were not dominating and driving the process that generates unintentional post-truth. We must assume our individual and collective responsibility for the emergence of unintentional post-truth that often enables intentional, malicious post-truth. 

We are both victims and victimizers. This approach gives us greater responsibility and also greater power. It is not enough to detect intentional post-truth, to identify fake news or to fact-check politicians. This is important, of course, but it will not be enough if we do not take a closer look at ourselves, challenge our ways of thinking and acting, and remain alert so we do not unwittingly make things worse.

Post-truth is both the product and the cause of an enormous divide. We live together in parallel narratives, in overlapping worlds where physics breaks down and we can pass by each other without having any mutual influence to deviate from the predetermined paths of what we have decided will be reality. This fracture, this discontinuity in the landscape, is a threat to the existence and development of human bonds, to our coexistence as a species on this planet, and thus to our survival. This is why post-truth is, fundamentally, a public health problem. 

This book is intended as a kick-start to break down the post-truth phenomenon into its main components so that together we can survive and prevail. It will focus on mechanisms and processes, illustrated through concrete examples, as a way to better prepare us to identify the structures that foster post-truth where they appear.

It is also part of a larger project that seeks to commit us to the truth, to make us more empowered citizens and to strengthen our mutual bonds. 

This does not mean that we will all hold a homogeneous position on the issues, far from it. But we should be able to have firm foundations common to all, agreements on what is true and what is not, so that we can build better societies and better protect democracy. 

One of the problems with post-truth is that it appears as an alternative to truth, as if truth were something we have, rather than an unknown destination on the horizon, one we need a compass to reach. If we feel lost, we can make a compass. If we do not agree on where the North is, no compass will help and we will be bound to wander following the erratic paths of ignorance. Or worse, bound to follow those who believe they have a North Star to guide them that they can relocate at their convenience.

We all share this one planet, this one reality. We also share concerns, problems and hopes. But we also have, and will continue to have, enormous differences among us. In order to discuss these different perspectives, we need to agree on the facts that we observe. Failing that, there is no possible exchange of ideas or arguments, no way to have shared experiences, and we run the risk of becoming indifferent to each other. 

Having a reality common to all is a basis that can allow us to agree or disagree on possible courses of action. But we will be together and in conversation, and that is the first step. Therefore, the fight against post-truth is also a fight to preserve the possibility of bonding as humans. 

We can try to better understand post-truth, especially to be able to identify it, confront it, and survive it (us and our species). The journey is long and hard, yes, but also very interesting and transformative. Embarking on it can not only teach us a lot about the world –and about ourselves-- but also give us the possibility to regain the power to be transformative agents and to reclaim the opportunity both to observe the world as it is in freedom and to use the best possible tools to imagine it as we want it to be, and thus help build it. 

What follows is an attempt to understand the problem of post-truth, to share both its importance and urgency, and to provide some specific tools to address it.