We have already identified two types of post-truth: the unintentional kind, which emerges from the behavior of each one of us, and the intentional kind, generated by people or interest groups who, in some way, learned to use, to hack, the mechanisms of unintentional post-truth for their own benefit. 

As a first step on the path that will allow us to recognize post-truth and confront it, we need to learn how to distinguish what is known from what is not. We need to answer the question: how do we know what we know? 

It’s time to build strong foundations: we will discuss how to identify and weigh the different kinds of evidence that we can obtain about factual issues in order to speak more clearly about the existence of truth in a practical sense, and we will also discuss how to evaluate consensus and factor in uncertainties. 

In an attempt to keep this section free from post-truth contamination, we will proceed with caution. The examples and stories we choose to illustrate these central points about how we know what we know will be uncontroversial. In the second section, we will address the main mechanisms that generate unintentional post-truth, and in the third, we will discuss some examples that show intentional post-truth in action. Finally, in the fourth section we will take stock of all of the above in order to look ahead and see how to survive post-truth. 

Linus Pauling was one of the most famous scientists of the 20th century. His discoveries about the nature of chemical bonds earned him the Nobel Prize in Chemistry in 1954. After the horror of the nuclear bombings of Hiroshima and Nagasaki, he became an important political activist and received the Nobel Peace Prize in 1962¹Only four people won more than one Nobel Prize, and only two of them in different fields: Linus Pauling and Marie Curie (who won them in Physics -1903- and Chemistry -1911-). .

So, when Pauling, in the late 1960s, championed the idea that colds can be prevented or shortened by taking vitamin C, many people paid attention. The sound of the fizzy orange tablet dissolving in water is a familiar childhood memory. Not that our parents were crazy, as it seemed reasonable to think that taking one or two grams of vitamin C a day protected against colds: Pauling, a man of remarkable intelligence and dedication, honored by the research community, believed it. He had to be right, right? 

Pauling founded an institute to research the benefits of vitamin C. And although it is not obvious, there is a problem here: when scientists design experiments, they do so to find out whether or not something is the way they imagine it to be, not to confirm it. Pauling was already convinced that vitamin C worked, even before conducting any experiments, and there is the problem: there is a huge difference between believing that something may be true, and being convinced that something is true before having concrete evidence. 

Pauling had started taking three grams of vitamin C daily. He was sure that he caught fewer colds, and - above all - he was convinced that this was due to the megadose of this vitamin (doses of more than one or two grams of vitamin C daily are already considered extremely large). With that conviction, and adding some research he had read on the subject, in 1970 he wrote a popular book that was very successful² See Pauling, L. (1970). Vitamin C and the Common Cold, San Francisco, W. H. Freeman. and that spread the idea that vitamin C was effective against colds. This kickstarted the industry of selling vitamin C supplements in pharmacies. A few years later, millions of people were following Pauling's advice. 

But Pauling was wrong. The opinions of scientists, unfortunately for some of them, have no more value than those of any other person. Opinions on factual matters are only valid when they are based on evidence. To obtain evidence, rule-based scientific research is required for reliable results. When this is the case, not only do they cease to be mere opinions, but it ceases to matter whose opinion it is (or whether or not they’re scientists). Science is a how, not a what, and much less, a who. 

Pauling's first experiments yielded results that seemed to support the vitamin’s efficacy. But around 1980, the first doubts began, based mainly on the fact that the methodology of Pauling's experiments was not entirely correct, and that the interpretations of the results were biased. A how, we said, not a what, much less a who. 

What could Pauling have done to confirm whether his suspicion was true in a less biased way? For starters, he could have compared a group of people taking the vitamin with another taking a pill that looked similar, but did not have the vitamin (a placebo). Moreover, since what was being assessed was the occurrence of colds - a disease with highly variable symptomatology across people - it would have been necessary for these groups to include many individuals (so as to lessen the impact of the particularities of each one), and for the study to be conducted for a sufficiently long period of time. If Pauling had still wanted to have more information, he should have repeated the experiment several times, and he would have obtained more or less the same result, which would have allowed him to assert that, indeed, vitamin C prevents colds. 

But he did none of this, or did it half-heartedly. Some of the methodological problems with Pauling's experiments were that they did not have a control group (of people receiving placebos), that the groups were too small, and that the results were not properly interpreted. 

Interested by the results, other institutes joined the research on the subject. The new studies were better and more rigorous methodologically. And this is where the big differences appeared: regardless of what they took, people had the same colds. At best, the vitamin group manifested less severe colds, but not much more than that. Pauling did not accept these results and never changed his position on the matter. The medical and scientific community, which took into account the quality and quantity of the available evidence, never endorsed him. 

On the one hand, we have the opinion of a genius, Linus Pauling. On the other, a mass of quality experimental evidence. Clearly, what we know is that there does not seem to be any justification for taking such high doses of vitamin C. The amount of vitamin C we need to be healthy may well be obtained from a proper diet rich in foods that contain it. 

However, these supplements are still sold in pharmacies and many continue to buy them. Why? Let's try to understand. 

Information comes to us through many different channels. We talk to neighbors, friends, relatives. We read newspapers, we watch television. We engage in social networks. We listen to experts, to our doctors, to our cultural icons, to celebrities. We also have our personal experience: the lessons we have learned in life. All this comes together to bombard us with opinions or ideas about the world that we do not always manage to evaluate carefully. Because, of course, information does not mean truth: there is good and bad quality information, there is true, probable, doubtful and false information, in a slippery slope that brings us closer and closer to post-truth, if we are not careful. 

Also, we consider our personal experience. If we tend to take vitamin C supplements and find that we get fewer colds than other people, we may tend to think that this is because vitamin C is protecting us. If a family member or friend tells us so, we might take that into account as well. And if a famous athlete or entertainer says the same thing, we may find it even more convincing. After all, celebrities often have access to the "best of the best" and, if they choose something, it's probably because it's wonderful. Right? 

The problem with positions that are based on personal experiences (one's own or others') is that there is no guarantee that they are not completely wrong.³More on this in Chapter VI. In all the cases mentioned above, we may encounter the same problems: How do we know if we are really getting sick less than other people? Are we considering how sick people are getting, or is it just a vague impression we have? And even if it were true that we are getting less sick, how can we know if it is due to the vitamin C we take as a supplement? Couldn't the cause be anything else, such as how much sleep we get, how exposed we are to cold-causing viruses, or what we eat? When we start to dissect what might be going on, we see that, beyond the feeling that something we do is working, we don't really have clear evidence that it is, unless we investigate it. 

Now, let's go up a step up the proverbial slope. It is no longer a friend or another person who tells us that since taking vitamin C they get fewer colds. Now, we read in a newspaper that a survey was carried out with 1000 people and that 82% of them consider that it is so, that vitamin C prevents colds. What now? Do we believe this any more? We might think that 820 people are a lot of people and that this has more weight than what our friend or neighbor told us. But this is not so. Reality is not voted. Democracy is not useful to find out if a fact is or is not a certain way. A survey is useful, on the other hand, as a method to find out people's opinion on a subject. In this case, it serves precisely to find out what they think about vitamin C. But an aggregate of opinions that are not based on evidence is in no way more than that: an aggregate of opinions. This is a stinging but key point: a survey is not a valid method for learning more about the facts of the natural world. Those 820 people may be just as wrong as our friend, neighbor, or celebrity about whether vitamin C does indeed prevent colds. 

 So? These people (a friend, a celebrity, or a survey) are not experts on the subject, but what they say usually has some impact on us⁴More on this in Chapter VIII.. Opinions, anecdotes or accounts of our own experiences are data that we take into account. And they may be useful. Or maybe not. From this information alone we cannot know.

Let's go one step further: what if the person recommending that we take vitamin C to avoid catching a cold is a doctor or an expert in another area relevant to this problem? And here is something important, so much so that it deserves to be said again: it is not so important who says what, but on what basis this person is telling us this. Is it based on personal experience? Since taking vitamin C do you find that you get fewer colds? If so, it is not in principle a different situation from what we were saying before, and we are looking at another Pauling (even if he has not yet won a Nobel Prize). We are back in the world of unsubstantiated opinions, of anecdotal cases. 

What if, instead, this idea is supported by some kind of evidence, more powerful evidence? The first thing would be to ask ourselves what this evidence might be. What would we look for as reliable data? If there is scientific research showing that vitamin C protects against colds through careful studies in which, for example, they compare how sick a group of people who don't take the supplements and another group who do get, we already have more reliable information. If our doctor speaks on the basis of information of this type, he is more reliable than if he read it in a magazine in which a famous person recommends taking vitamin C, or if he speaks from his own personal, anecdotal experience, just like our fictitious friend or the 820 people in the survey.⁵More on this in Chapter VI.

Yes, a doctor's opinion, as was the case with Pauling's opinion, is worth the same as anyone else's when it comes to scientific matters: nothing, unless it is supported by evidence. It is not a question of intelligence or degrees. Few more educated or more intelligent than Pauling, few were more wrong than he was on this subject. 

Yet, not only are there misconceptions, but some, in a junior version of post-truth, exploit ambiguity or even lack of information about how we choose which information to trust. The advertising industry (9 out of 10 advertisers, according to a survey by the University of Andwhoareyou) exploits this confusion at its convenience all the time, parading dentists, nutritionists or doctors recommending certain products without it being clear whether or not those products are really effective or if they have gone through a methodologically valid process to support their statements. 

So what characteristics should we expect in an assertion in order to have more confidence in it? We need it to be supported by quality evidence. For example, if we read a scientific paper directly, or if an expert tells us about it, or if we read it (properly exposed) in the media, we can trust it more than if this information did not exist and we only had, at best, opinions or personal experiences. There are no hard and fast rules, but we can adopt some "rules of thumb" to guide us. What’s crucial is that we begin to see confidence in an assertion as a continuum and not as black or white⁶More on this in Chapter IV.. It is not a matter of being sure or not sure, but of being a little more or a little less sure as new evidence emerges. 

In assessing whether vitamin C prevents colds or not, we had to take into account the type and quality of scientific evidence. It was not enough that our parents had given us the supplements when we were children, nor the opinion of Linus Pauling, even though he was a two-time Nobel Prize winner, much loved and admired by his colleagues. Pauling was an extremely intelligent and capable man who, just on this particular subject, was completely wrong. Because this Nobel laureate was also a human being with the liability to make mistakes that all human beings have. It is precisely because we can - and usually do - make mistakes that we need a methodology to understand what is evidence and what is merely unfounded opinion. 

We accept, then, that there is an "out there", a real world. But we also know that we cannot rely too much on our personal experience and intuitions because they are often wrong. In order to get answers, we need to approach the questions by looking for concrete and reliable evidence. Fortunately, science provides us with the tools to examine our beliefs as far outside ourselves as possible⁷Or factor in our biases when measuring, if we prefer to think of it that way..

When we think of science, we may remember high school subjects in which we were required to learn facts by rote or perform procedures mechanically without really understanding them: knowing the characteristics that describe arthropods, balancing chemical equations, or stating Newton's laws. It is also possible that the word science evokes a type of media story, such as when a new satellite is sent into space or a drug is discovered that could cure a disease. But that's the least fun and interesting side of it: the best thing about science is that it is much more than results. It is fundamentally a methodology, a set of mental tools. It is a process, an action, a verb. 

If tomorrow we were to lose all the scientific knowledge we have, we could probably recover it in a couple of generations. If we were to irreversibly lose the method of science, we would be condemned to have our knowledge stop where it stands, and to be unable to understand the world. It is the difference between having a piece of bread to eat and knowing how to make bread. 

Of course, science is not enough to "save us from post-truth". Later, when we discuss the specific mechanisms that generate post-truth, we will see that it is not enough that the information exists, that we know it, and that we understand it. But we need solid foundations before we can begin to analyze post-truth. We need to see clearly that the methodology that science has for answering questions helps us to elucidate what is true and what is not. 

 Unless explicitly clarified otherwise, when we talk about sciences, we will refer only to those that share the methodology of making observations or experiments that allow us to find out how certain real-world phenomena occur. We will focus on the way in which the questions are answered and not so much on the content of those questions. Thus, this use of the word includes the natural sciences, i.e., those that study natural phenomena, as well as the study of other problems that do not seem to be so clearly part of science in terms of content, but which do apply scientific methodology, for example, in order to find out whether or not a new drug is effective. We will not discuss here what happens with the non-empirical branches of mathematics, nor what is related to technology, for example. With respect to the social sciences, there are areas in which this methodological approach applies, such as econometrics or experimental psychology, and others in which it does not. When discussing how to approach the truth, the subject is irrelevant. What we need to start with is a better understanding of science understood as methodology, as a way of approaching knowledge, and not as a disciplinary content. As Karl Popper said, "classification into disciplines is comparatively unimportant. We are students of problems, not of disciplines".⁸But this does not affect my point that the classification into disciplines is comparatively unimportant, and that we are students not of disciplines but of problems. Karl Popper, Conjectures and Refutations. 

In this book, we will call scientific knowledge which can be obtained with this methodology, regardless of the subject. This set of tools allows us to find out whether an idea we have about the world reasonably corresponds to reality or not. Science, understood as answering questions with this kind of evidence-seeking gaze, is beginning to permeate almost everything around us. 

In order to get closer to the truth it is important to understand how reliable knowledge is generated. Knowledge comes mostly, but not exclusively, from scientific activity understood as a particular methodology that generates evidence and interprets it with a greater or lesser degree of reliability. 

Regarding vitamin C, we came to a solution, to an answer, to something we can say we know: the scientific evidence, which is abundant and highly reliable, does not support the idea that vitamin C is effective against colds.⁹See Hemiliä, H. and Chalker, E. (2013). Vitamin C for the prevention and treatment of the common cold, Cochrane Database of Systematic Review, no. 5.  This is the truth on this subject, and if anyone still maintains that vitamin C works, it is just an unfounded opinion and not much more. 

But, after so much work by so many people, we only have this solution and no other. We don't know whether or not other vitamins work for what is promised, or if vitamin C is great for anything else, nor do we know anything about much else. We are running after problems and we still don't know whether or not there is anything we can do to move around better in this world full of information, which sometimes is correct and consistent and other times is irrelevant, incomplete or downright contradictory. 

The subject of knowledge is as slippery as the subject of truth. Philosophy has many definitions of what it is to know. Our position on this is the same as the one we took to define truth: something practical, provisional, gradual, and always subject to revision in the light of evidence. In this framework, if we want to know, we need to be able to find those few reliable statements -that is, those supported by evidence- that seem to be lost in that sea of knowledge. Not only must we find them, but we must also accept them so as not to surrender to post-truth. But how to achieve this? 

James Randi is one of the most famous magicians and escapologists of the 20th century. He is not a magician in books, like Harry Potter, or movies, like Dr. Strange. 

He has no superpowers and, more importantly, he doesn't claim to have them. Inspired by the work of Harry Houdini, Randi dedicated himself to stage illusionism. For half a century, he gave performances in theaters and on television that made him very popular. What he knows, and knows very well, is how to trick us into believing that his magic is "real". 

When we see any magic trick, we know that the person in front of us does not really have supernatural powers. We know we are being tricked, but, if the magician is good, we do not understand how. And that is what we enjoy as a performance: the surprise of something seemingly impossible taking place before our very eyes. That is the unspoken contract between the magician and the audience: we are going to suspend disbelief for a while and allow ourselves to enjoy the amazement, but always keeping in mind that this is a show, and the magician, a performer. But some break this pact and claim to have paranormal powers, and they come in all forms, with different discourses and in all historical contexts. 

Uri Geller is an illusionist who, in the 1970s, claimed to have real powers. In his many appearances, he would show how he could bend spoons with his mind or guess what someone was thinking. But what's the fun in doing tricks if you're simultaneously convincing the other person that it's not something you achieve through skill, but by having supposed powers? For Randi, and for almost all other professional magicians, what is fascinating is that those who watch them try, unsuccessfully, to realize how they are managing to escape from a small space, bound with chains and handcuffs, or how they are doing those tricks with coins, handkerchiefs or doves. Therein lies the art and pleasure of magic: that the audience knows it is not true and, at the same time, believes (for a while) that it is. As soon as a magician pretends to convince his audience that his powers are real, that he can make objects appear and disappear, or that he can divine thought, he becomes a fraud. James Randi, in parallel with his own career as an illusionist, began to devote himself almost professionally to exposing Uri Geller and other so-called "psychics" by repeating their tricks on television and explaining them to the audience. He also revealed the tricks of pastors who claimed to communicate with God and to have powers, and who convinced their followers to abandon treatments for cancer and other illnesses and instead heal themselves by praying and donating money, curiously enough, to those same pastors. 

Thus, Randi managed to prevent several of these pastors from continuing to profit from their fraudulent actions. Unmasking those who live at the expense of people's vulnerabilities is, however, problematic: those who expose "prophets", "psychics" or people with "paranormal powers" are always trying to catch up with them. Similarly, those who try to refute with very specific arguments alleged medical treatments that do not work, explaining in detail why not, or who try to demonstrate why certain statements, whatever the subject, are false, are always one step behind. Alberto Brandolini formulated the "principle of asymmetry of nonsense" as "the amount of energy needed to refute nonsense is an order of magnitude greater than that needed to generate it".¹⁰The bullshit asymmetry: the amount of energy needed to refute bullshit is an order of magnitude bigger than to produce it. It is very easy and quick to tell nonsense or a lie. On the other hand, gathering convincing evidence to destroy it is more difficult and time-consuming. It's a race lost before it even begins, but it's a race that still needs to be run.

 This presents a conundrum: proper research is essential for us to obtain reliable evidence, but as society we need to make decisions in the meantime, and sometimes we cannot wait long. 

How can we combat these frauds more effectively? How can we stay one step ahead? In the words of Randi himself, who soon realized that he could not cope with exposing all these charlatans, "if I explain the tricks, I would give you a solution, but I would not give you all the solutions". 

The same happens not only with frauds, but with knowledge in general. Every new piece of knowledge related to factual matters is achieved in a particular, meticulous and demanding way, and it is indispensable that this be so. But how do we, who want to use that knowledge to make better decisions, find it and identify it? Is there a shortcut, or are we condemned to evaluate statement by statement, to see if it is reliable or not? 

We need a "solution for all solutions" and we need it urgently so that we do not sink in the sea of irrelevant or inconsistent pieces of information while we try to identify those that are valuable and more reliable. It is not enough to chase the claims that appear in the media or on social networks and trying to find out whether it is true or not that coffee causes cancer, that immigrants or refugees are criminals, that vaccines work, that the horoscope can tell what will happen to us in the week ahead, or that taxing sugary drinks helps prevent diabetes.¹¹More on this in Chapter XIV. How could we, instead, provide a more general solution, with certain rules that can be applied to new situations? 

We can try to fact-check everything, but as a mechanism it is very time consuming and difficult, requiring dedication and a certain level of expertise. We expect journalists or news agencies to do this before publishing their stories, and as a society we should demand it. We can also take the lead and be demanding before we take information at face value and repeat it as valid.

 On the positive side, there are reliable information generators with guides on how to do good fact-checking, as well as lists of unreliable Internet sites that have spread fake news. But fact-checking falls short as a solution for us, the general public. It is valuable, of course, but insufficient. Lists of untrustworthy sites become instantly useless, because whoever wants to spread falsehoods can generate a new site in a snap. Moreover, what stops them from compiling reliable Internet sites and saying that they are not reliable? 

We do not want to be given fish, we want to learn how to fish and thus become more independent. Fact-checking will not protect us completely from the abundant fake news that is very easily distributed through social networks. It may be able to identify some, but not all of it, not fast enough, and it will never have the same outreach. 

Professional research, which allows us to find and interpret the evidence in order to evaluate claims, is essential. Professional journalism, communicating assertions properly after verifying their veracity, is also essential. But even so, we need another tool: at some point, each of us must become an active agent, a screener of reliable information. The methodology of science, as a way of answering questions in many areas of knowledge, can be useful for this. It is not the only tool, but it is important for building solid foundations for truth. Later, when we develop a more complete and complex picture, we will see that, while we need to better understand the methodology of science, more is needed to combat post-truth. 

To find Randi’s solution to all solutions and not get lost in a stormy sea of confusing or fraudulent information, we need to take a closer look, in order to better understand how we know what we know and to what extent we know it. This will help us distinguish the true from the false and the truth from the lie, the first step to fight the post-truth epidemic. 

Throughout this first section on how we know what we know we will delve into the world of evidence: what it is, how reliable it is, how much it affects our imagination and our biases, and what consensus or its lack means.