In 2002 and early 2003, the US government repeatedly assured everyone that its planned invasion of Iraq would go swimmingly. “The people will be enormously relieved and liberated,” said Donald Rumsfeld. “I think it will go relatively quickly… weeks rather than months,” said Dick Cheney. Six months into the occupation, Richard Perle announced: “A year from now, I’ll be very surprised if there is not some grand square in Baghdad that is named after President Bush.” History does not record the subsequent level of his amazement when such an honour failed to materialise. But the question that can still nag today is: how did they get it so wrong?

The perceived rise in “fake news” over the past few years is often discussed as though it were a natural phenomenon, like an increase in the frequency of earthquakes. Commentators have focused primarily on how the consumers of fake news might arm themselves against it. But what if the thing that really needs to change is the way the masters of disinformation think? The philosopher Quassim Cassam introduces his tightly argued book with the question of how the architects of the Iraq War convinced themselves that it would be such a triumph, and his answer is that they were personally, and culpably, defective in reason: they suffered from “vices of the mind”, which include “arrogance, imperviousness to evidence, and an inability to deal with mistakes”. These he defines as “epistemic vices”, because they are to do with one’s attitude to acquiring and maintaining knowledge.

This is a more granular, and useful, diagnosis than the simple assumption that the Bush regime were all inveterate liars, or stupid. Rumsfeld in particular was many things, but he was not stupid. And instead of calling him a liar, we may more productively say of him what Cassam later says so persuasively of Boris Johnson, that he exhibits “epistemic insouciance”. This is “an indifference or lack of concern with respect to whether [his] claims are grounded in reality or the evidence”. The concept is related to the philosopher Harry Frankfurt’s portrait of the bullshitter, who differs from the liar in that he doesn’t care whether what he says is true or not. And so, Cassam explains, “Epistemic insouciance is the attitude… that makes one a bullshitter and thereby causes one to spout bullshit.” But the epistemically insouciant man, he argues, is not simply indifferent to truth and evidence, he holds an active contempt for them: “Contempt for the truth, contempt for experts, and, in the case of politicians, contempt for the public.”

Boris Johnson certainly seems to belong in the picture dictionary under this heading; but I did worry that he would be irritatingly pleased to be described as “insouciant”, which in English sometimes means merely “careless” but more often aligns with the admiring French sense of “carefree”.

If one is simply too lazy to find out the truth, by contrast, one is a “slacker”. Some serious philosophical work has already been done on this concept, concluding that “slackers don’t care enough about serious inquiry to feel contempt for it”. (This might usefully label, for instance, Johnson’s erstwhile cabinet colleague, David Davis.) And if you actively oppose knowledge, you are an example of “epistemic malevolence”, which describes tobacco and oil-company lobbyists deliberately sowing lies about whether cigarettes cause cancer or fossil fuels cause global warming.

This book is excellent at such teasing-apart of conceptual differences. The general reader might find it occasionally dry, but that is the way with analytic philosophy, and I suspected a few times that the pedantry was deliciously deadpan: “If one needs to be epistemically competent in order to determine one’s level of epistemic competence then the epistemically incompetent are in a particularly bad position to know that they are epistemically incompetent.” This is Cassam’s amusingly technical rephrasing of the Dunning-Kruger effect, which says that in many fields “incompetent individuals lack the skills needed to know that they are incompetent”: hence the incurable arrogance that so often goes hand in hand with ineptitude.

I suggested earlier that Rumsfeld, Cheney and co “suffered” from vices of the mind, but such men don’t actually appear to be caused any distress by their own intellectual degradation. And this might be because of the Dunning-Kruger effect, which reassuringly cloaks from them their own deficiencies. Cassam expands this idea by noting that many of his epistemic vices are “stealthy”, in that having them seems to preclude – or at least militate against – coming to know that one has them. One example of a stealthy epistemic vice is dogmatism, or closed-mindedness. In order to decide that you need to be more open-minded, it seems that you already have to have opened your mind a little bit, somehow or other. So the truly closed-minded person will never realise he is closed-minded.

But what if closed-mindedness is not thoroughly a bad thing anyway? It is often remarked that a little bit of dogmatism can do you good. The philosopher of science Thomas Kuhn, for instance, said that during phases of “normal science” productive work is done by those who are dogmatic about the correctness of current theory and ignore anything that seems too outlandish – until the outlandish becomes unignorable, and a scientific revolution occurs.

Another of Cassim’s examples is that, since one knows the Holocaust happened, one could treat any and all material claiming that it didn’t as garbage not worth one’s time. On this subject, though, Cassam takes a firm line. He redefines the scientific “dogmatism” of Kuhn as not really dogmatism but “firmness or tenacity”. Cassam argues that while being closed-minded to Holocaust denial or other conspiracy theories may be understandable, it is “nothing to be proud of”. After all, he points out, one can learn more about the truth from careful refutations of lies: as happened, for instance, at the libel trial of Deborah Lipstadt, which concluded that the plaintiff David Irving really was a Holocaust denier.

But who has the time to expose themselves to the enormous cesspool of such material available nowadays on the off-chance they might learn something? And, contrarily, who has the time to fact-check everything they see online? Cassam acknowledges such an objection, but is curiously optimistic about how it might be overcome. Anyone with internet access and “an attention span of more than five minutes”, he writes, can discover that there is “something fishy” about Irving’s view of history. But to do so requires a set of skills – source evaluation, critical thinking and so on – that someone may lack, without it being their fault. This is why people with an attention span of much longer than five minutes can easily get sucked by profit-maximising video-recommendation algorithms down the vortex of fake-news doom, ending up where an interest in conspiracy theories always ends up: anti-Semitism and Holocaust denial.

Indeed, this sort of journey can and does happen to people who are basically kind and decent. (I have known some of them.) The failure of our public conversation to acknowledge this fact is itself an epistemic or at least broadly intellectual vice: it is a failure of moral imagination.

The question, then, is whether such unfortunate citizens, in addition to leaders who ought to know better, should really be described as having intellectual “vices” at all. Cassam argues that they should because, even if you are not responsible for acquiring such a vice (through miseducation or a simple lack of time), you retain “revision responsibility” for it, in that it’s possible to rectify such vices once they are recognised. But if they are stealthy vices, this might not ever happen. So it remains debatable how far people who become mentally entrapped by racist or anti-vaccination propaganda, say, deserve to be personally blamed for what Cassam would identify as their epistemic vice of “gullibility” – which could as well be reframed as an idealistic surfeit of trust in our fundamentally broken information economy.

Cassam knows that the demands he places on individuals to rectify their own vices are only justified if they have a certain basic level of intellectual competence. “The only hope for a society that cares about knowledge,” he therefore says, “is to equip its citizens with the intellectual and other means to distinguish truth from lies.”

This is right, but it is also the philosophical equivalent of unicorns for all. How will a society so equip itself if the people at the very top are, as Cassam has so amply demonstrated, themselves riddled with the worst epistemic vices, as well as instituting polices that lead to mass library closures and other ways of intellectually immiserating a captive populace?

At the end of this superb (and icily furious) book, one is persuaded by the author’s diagnosis, but a cure looks to be as far off as ever.

Vices of the Mind: From the Intellectual to the Political

Quassim Cassam

Oxford University Press, 224pp, £25