Meet Oliver. Like many of his friends, Oliver thinks he is an expert on 9/11. He spends much of his spare time looking at conspiracist websites and his research has convinced him that the terrorist attacks on New York and Washington, DC, of 11 September 2001 were an inside job. The aircraft impacts and resulting fires couldn’t have caused the Twin Towers of the World Trade Center to collapse. The only viable explanation, he maintains, is that government agents planted explosives in advance. He realises, of course, that the government blames Al-Qaeda for 9/11 but his predictable response is pure Mandy Rice-Davies: they would say that, wouldn’t they?

Polling evidence suggests that Oliver’s views about 9/11 are by no means unusual. Indeed, peculiar theories about all manner of things are now widespread. There are conspiracy theories about the spread of AIDS, the 1969 Moon landings, UFOs, and the assassination of JFK. Sometimes, conspiracy theories turn out to be right – Watergate really was a conspiracy – but mostly they are bunkum. They are in fact vivid illustrations of a striking truth about human beings: however intelligent and knowledgeable we might be in other ways, many of us still believe the strangest things. You can find people who believe they were abducted by aliens, that the Holocaust never happened, and that cancer can be cured by positive thinking. A 2009 Harris Poll found that between one‑fifth and one‑quarter of Americans believe in reincarnation, astrology and the existence of witches. You name it, and there is probably someone out there who believes it.

You realise, of course, that Oliver’s theory about 9/11 has little going for it, and this might make you wonder why he believes it. The question ‘Why does Oliver believe that 9/11 was an inside job?’ is just a version of a more general question posed by the US skeptic Michael Shermer: why do people believe weird things? The weirder the belief, the stranger it seems that someone can have it. Asking why people believe weird things isn’t like asking why they believe it’s raining as they look out of the window and see the rain pouring down. It’s obvious why people believe it’s raining when they have compelling evidence, but it’s far from obvious why Oliver believes that 9/11 was an inside job when he has access to compelling evidence that it wasn’t an inside job.

I want to argue for something which is controversial, although I believe that it is also intuitive and commonsensical. My claim is this: Oliver believes what he does because that is the kind of thinker he is or, to put it more bluntly, because there is something wrong with how he thinks. The problem with conspiracy theorists is not, as the US legal scholar Cass Sunstein argues, that they have little relevant information. The key to what they end up believing is how they interpret and respond to the vast quantities of relevant information at their disposal. I want to suggest that this is fundamentally a question of the way they are. Oliver isn’t mad (or at least, he needn’t be). Nevertheless, his beliefs about 9/11 are the result of the peculiarities of his intellectual constitution – in a word, of his intellectual character.

Usually, when philosophers try to explain why someone believes things (weird or otherwise), they focus on that person’s reasons rather than their character traits. On this view, the way to explain why Oliver believes that 9/11 was an inside job is to identify his reasons for believing this, and the person who is in the best position to tell you his reasons is Oliver. When you explain Oliver’s belief by giving his reasons, you are giving a ‘rationalising explanation’ of his belief.

The problem with this is that rationalising explanations take you only so far. If you ask Oliver why he believes 9/11 was an inside job he will, of course, be only too pleased to give you his reasons: it had to be an inside job, he insists, because aircraft impacts couldn’t have brought down the towers. He is wrong about that, but at any rate that’s his story and he is sticking to it. What he has done, in effect, is to explain one of his questionable beliefs by reference to another no less questionable belief. Unfortunately, this doesn’t tell us why he has any of these beliefs. There is a clear sense in which we still don’t know what is really going on with him.

Now let’s flesh out Oliver’s story a little: suppose it turns out that he believes lots of other conspiracy theories apart from the one about 9/11. He believes the Moon landings were faked, that Diana, Princess of Wales, was murdered by MI6, and that the Ebola virus is an escaped bioweapon. Those who know him well say that he is easily duped, and you have independent evidence that he is careless in his thinking, with little understanding of the difference between genuine evidence and unsubstantiated speculation. Suddenly it all begins to make sense, but only because the focus has shifted from Oliver’s reasons to his character. You can now see his views about 9/11 in the context of his intellectual conduct generally, and this opens up the possibility of a different and deeper explanation of his belief than the one he gives: he thinks that 9/11 was an inside job because he is gullible in a certain way. He has what social psychologists call a ‘conspiracy mentality’.

The gullible rarely believe they are gullible and the closed-minded don’t believe they are closed-minded

Notice that the proposed character explanation isn’t a rationalising explanation. After all, being gullible isn’t a reason for believing anything, though it might still be why Oliver believes 9/11 was an inside job. And while Oliver might be expected to know his reasons for believing that 9/11 was an inside job, he is the last person to recognise that he believes what he believes about 9/11 because he is gullible. It is in the nature of many intellectual character traits that you don’t realise you have them, and so aren’t aware of the true extent to which your thinking is influenced by them. The gullible rarely believe they are gullible and the closed-minded don’t believe they are closed-minded. The only hope of overcoming self-ignorance in such cases is to accept that other people – your co-workers, your spouse, your friends – probably know your intellectual character better than you do. But even that won’t necessarily help. After all, it might be that refusing to listen to what other people say about you is one of your intellectual character traits. Some defects are incurable.

Gullibility, carelessness and closed-mindedness are examples of what the US philosopher Linda Zagzebski, in her book Virtues of the Mind (1996), has called ‘intellectual vices’. Others include negligence, idleness, rigidity, obtuseness, prejudice, lack of thoroughness, and insensitivity to detail. Intellectual character traits are habits or styles of thinking. To describe Oliver as gullible or careless is to say something about his intellectual style or mind-set – for example, about how he goes about trying to find out things about events such as 9/11. Intellectual character traits that aid effective and responsible enquiry are intellectual virtues, whereas intellectual vices are intellectual character traits that impede effective and responsible inquiry. Humility, caution and carefulness are among the intellectual virtues Oliver plainly lacks, and that is why his attempts to get to the bottom of 9/11 are so flawed.

Oliver is fictional, but real-world examples of intellectual vices in action are not hard to find. Consider the case of the ‘underwear bomber’ Umar Farouk Abdulmutallab, who tried to blow up a flight from Amsterdam to Detroit in 2009. Abdulmutallab was born in Lagos, Nigeria, to affluent and educated parents, and graduated from University College London with a degree in mechanical engineering. He was radicalised by the online sermons of the Islamic militant Anwar al-Awlaki, who was subsequently killed by an American drone strike. It’s hard not to see the fact that Abdulmutallab was taken in by Awlaki’s sermons as at least partly a reflection of his intellectual character. If Abdulmutallab had the intellectual character not to be duped by Awlaki, then perhaps he wouldn’t have ended up on a transatlantic airliner with explosives in his underpants.

Intellectual character explanations of questionable beliefs are more controversial than one might imagine. For example, it has been suggested that explaining peoples’ bad behaviour or weird beliefs by reference to their character makes us more intolerant of them and less empathetic. Yet such explanations might still be correct, even if they have deleterious consequences. In any case, it’s not obvious that character explanations should make us less tolerant of other peoples’ foibles. Suppose that Oliver can’t help being the kind of person who falls for conspiracy theories. Shouldn’t that make us more rather than less tolerant of him and his weird beliefs?

A different objection to character-based explanations is that it’s just not true that people have questionable beliefs because they are stupid or gullible. In How We Know What Isn’t So (1991), the US social psychologist Thomas Gilovich argues that many such beliefs have ‘purely cognitive origins’, by which he means that they are caused by imperfections in our capacities to process information and draw conclusions. Yet the example he gives of a cognitive explanation takes us right back to character explanations. His example is the ‘hot hand’ in basketball. The idea is that when a player makes a couple of shots he is more likely to make subsequent shots. Success breeds success.

Gilovich used detailed statistical analysis to demonstrate that the hot hand doesn’t exist – performance on a given shot is independent of performance on previous shots. The question is, why do so many basketball coaches, players and fans believe in it anyway? Gilovich’s cognitive explanation is that belief in the hot hand is due to our faulty intuitions about chance sequences; as a species, we’re bad at recognising what genuinely random sequences look like.

And yet when Gilovich sent his results to a bunch of basketball coaches, what happened next is extremely revealing. One responded: ‘Who is this guy? So he makes a study. I couldn’t care less.’ This seems like a perfect illustration of intellectual vices in operation. The dismissive reaction manifested a range of vices, including closed-mindedness and prejudice. It’s hard not to conclude that the coach reacted as he did because he was closed-minded or prejudiced. In such cases as this, as with the case of Oliver, it’s just not credible that character traits aren’t doing significant explanatory work. A less closed-minded coach might well have reacted completely differently to evidence that the hot hand doesn’t exist.

Could we explain the dismissiveness of the coach without referring to his personality in general? ‘Situationists’, as they are called, argue that our behaviour is generally better explained by situational factors than by our supposed character traits. Some see this as a good reason to be skeptical about the existence of character. In one experiment, students at a theological seminary were asked to give a talk elsewhere on campus. One group was asked to talk about the parable of the Good Samaritan, while the rest were assigned a different topic. Some were told they had plenty to time to reach the venue for the lecture, while others were told to hurry. On their way to the venue, all the students came across a person (an actor) apparently in need of help. In the event, the only variable that made a difference to whether they stopped to help was how much of a hurry they were in; students who thought they were running late were much less likely to stop and help than those who thought they had time. According to the Princeton philosopher Gilbert Harman, the lesson of such experiments is that ‘we need to convince people to look at situational factors and to stop trying to explain things in terms of character traits’.

You say that Oliver is gullible for believing his 9/11 conspiracy theory; he retorts that you are gullible for believing the conclusions of the 9/11 Commission

The character traits that Harman had in mind are moral virtues such as kindness and generosity, but some situationists also object to the idea of intellectual virtues and vices. For example, they point to evidence that people perform much better in problem-solving tasks when they are in a good mood. If trivial situational factors such as mood or hunger are better at explaining your intellectual conduct than your so-called intellectual character, then what is the justification for believing in the existence of intellectual character traits? If such traits exist, then shouldn’t they explain one’s intellectual conduct? Absolutely, but examples such as Oliver and Gilovich’s basketball coach suggest that intellectual character traits do explain a person’s intellectual conduct in an important range of cases. People don’t believe weird things because they are hungry or in a bad (or good) mood. The view that people don’t have character traits such as gullibility, carelessness or prejudice, or that people don’t differ in intellectual character, deprives us of seemingly compelling explanations of the intellectual conduct of both Oliver and the basketball coach.

Suppose it turns out that Oliver lives in a region where conspiracy theories are rife or that he is under the influence of friends who are committed conspiracy theorists. Wouldn’t these be perfectly viable situational, non-character explanations of his beliefs about 9/11? Only up to a point. The fact that Oliver is easily influenced by his friends itself tells us something about his intellectual character. Where Oliver lives might help to explain his beliefs, but even if conspiracy theories are widespread in his neck of the woods we still need to understand why some people in his region believe them, while others don’t.

Differences in intellectual character help to explain why people in the same situation end up believing such different things. In order to think that intellectual character traits are relevant to a person’s intellectual conduct, you don’t have to think that other factors, including situational factors, are irrelevant. Intellectual character explains intellectual conduct only in conjunction with a lot of other things, including your situation and the way your brain processes information. Situationism certainly would be a problem for the view that character traits explain our conduct regardless of situational factors, but that is not a view of character anyone has ever wanted to defend.

In practical terms, one of the hardest things about dealing with people such as Oliver is that they are more than likely to accuse you of the same intellectual vices that you detect in them. You say that Oliver is gullible for believing his 9/11 conspiracy theory; he retorts that you are gullible for believing the conclusions of the 9/11 Commission. You say that he dismisses the official account of 9/11 because he is closed-minded; he accuses you of closed-mindedness for refusing to take conspiracy theories seriously. If we are often blind to our own intellectual vices then who are we to accuse Oliver of failing to realise that he believes his theories only because he is gullible?

These are all legitimate questions, but it’s important not to be too disconcerted by this attempt to turn the tables on you. True, no one is immune to self-ignorance. That doesn’t excuse Oliver. The fact is that his theory is no good, whereas there is every reason to believe that aircraft impacts did bring down the Twin Towers. Just because you believe the official account of what happened in 9/11 doesn’t make you gullible if there are good reasons to believe that account. Equally, being skeptical about the wilder claims of 9/11 conspiracy theorists doesn’t make you closed-minded if there are good reasons to be skeptical. Oliver is gullible because he believes things for which he has no good evidence, and he is closed-minded because he dismisses claims for which there is excellent evidence. It’s important not to fall into the trap of thinking that what counts as good evidence is a subjective matter. To say that Oliver lacks good evidence is to draw attention to the absence of eye-witness or forensic support for his theory about 9/11, and to the fact that his theory has been refuted by experts. Oliver might not accept any of this but that is, again, a reflection of his intellectual character.

the aims of education should include cultivating intellectual virtues and curtailing intellectual vices

Once you get past the idea that Oliver has somehow managed to turn the tables on you, there remains the problem of what to do about such people as him. If he is genuinely closed-minded then his mind will presumably be closed to the idea that he is closed-minded. Closed-mindedness is one of the toughest intellectual vices to tackle because it is in its nature to be concealed from those who have it. And even if you somehow get the Olivers of this world to acknowledge their own vices, that won’t necessarily make things any better. Tackling one’s intellectual vices requires more than self-knowledge. You also need to be motivated to do something about them, and actually be able to do something about them.

Should Oliver be condemned for his weaknesses? Philosophers like to think of virtues as having good motives and vices as having bad motives but Oliver’s motives needn’t be bad. He might have exactly the same motivation for knowledge as the intellectually virtuous person, yet be led astray by his gullibility and conspiracy mentality. So, both in respect of his motives and his responsibility for his intellectual vices, Oliver might not be strictly blameworthy. That doesn’t mean that nothing should be done about them or about him. If we care about the truth then we should care about equipping people with the intellectual means to arrive at the truth and avoid falsehood.

Education is the best way of doing that. Intellectual vices are only tendencies to think in certain ways, and tendencies can be countered. Our intellectual vices are balanced by our intellectual virtues, by intellectual character traits such as open-mindedness, curiosity and rigour. The intellectual character is a mixture of intellectual virtues and vices, and the aims of education should include cultivating intellectual virtues and curtailing intellectual vices. The philosopher Jason Baehr talks about ‘educating for intellectual virtues’, and that is in principle the best way to deal with people such as Oliver. A 2010 report to the University College London Council about the Abdulmutallab case came to a similar conclusion. It recommended the ‘development of academic training for students to encourage and equip them not only to think critically but to challenge unacceptable views’. The challenge is to work out how to do that.

What if Oliver is too far gone and can’t change his ways even if he wanted to? Like other bad habits, intellectual bad habits can be too deeply entrenched to change. This means living with their consequences. Trying to reason with people who are obstinately closed-minded, dogmatic or prejudiced is unlikely to be effective. The only remedy in such cases is to try to mitigate the harm their vices do to themselves and to others.

Meanwhile, those who have the gall to deliver homilies about other peoples’ intellectual vices – that includes me – need to accept that they too are likely very far from perfect. In this context, as in most others, a little bit of humility goes a long way. It’s one thing not to cave in to Oliver’s attempt to turn the tables on you, but he has a point at least to this extent: none of us can deny that intellectual vices of one sort or another are at play in at least some of our thinking. Being alive to this possibility is the mark of a healthy mind.