The Heretics: Adventures with the Enemies of Science, By Will Storr

David Irving went to prison for insisting that the Holocaust never took place, and even if it did it wasn't as bad as has been made out – and in any case Hitler didn't know about it. At least, says Irving, there is no written evidence that Hitler ever ordered the extermination of the Jews – and without such evidence, it's all rumour.

But in a speech on 30 January 1939, Hitler said that "if the international finance Jewry… succeed once more in plunging nations into world war… the consequence [will be] the annihilation of the Jewish race in Europe". Ah, said Irving, when Will Storr confronted him, Hitler was merely predicting – not recommending. In any case, Hitler didn't say "annihilate". He used the word ausrotten. Nowadays ausrotten does mean "exterminate" – but in 1939 it simply meant "extirpate", root out; which, says Irving, is not the same thing at all.

Irving's scholarship is impressive, as other historians largely agree. But Storr is scholarly too, and sent off for a Chambers' 1939 German-English dictionary. It says: "Ausrott -en, v.a. extirpate, exterminate, root out. Comp. – ungs -fried m war of extermination". In 1939, in short, whether by way of recommendation or prophecy, Hitler did say "exterminate". On this significant point of detail Irving the scholar is wrong.

So is Irving a liar, or simply less assiduous than he often appears? Neither, says Storr. The truth is far more subtle and disturbing. For although Irving epitomises what most people mean by "heretic" – promulgating ideas that seem way off-beam and potentially dangerous – both his obsession and his misrepresentations reflect, in extreme and unprepossessing form, how human minds in general really work.

Our brains – and those of other people whom we admire as intellectuals – are far more Irving-like than we might care to believe. For Irving above all is a story-teller. Steered as all of us must be by his genes and upbringing, he has created his own worldview, his own account of how things are. Once the personal narrative is properly worked out then nothing, not even black-and-white evidence, can shift him. Once he had it in his head that Hitler never mentioned extermination, the mere fact that he did would not lead him to change his story.

But we are all condemned to live entirely in our own heads. What we know of the outside world is compounded from our inherited preconceptions and from streams of codified electrical pulses that flow into our various receptor centres: 33 dedicated to vision alone. Our brains, and sense organs, perforce are highly selective: for we are assailed with an estimated 14 million bits of information per second of which we can register only a minute proportion. Our impression of reality is, in truth, a master-class in re-invention.

From birth to age two our brains are wonderfully flexible – adding another 1.4 million synapses per second; eager to learn more. But as we mature, we enter consolidation mode. We turn what we have learnt into a coherent story. Once the story is nicely rounded out our brains and our minds seem to say, "Enough's enough!" Then, new impressions are either ignored or re-shaped to fit what we think we already know. Politicians and academics like to tell us that their policies and theories are "evidence-based", yet they, perhaps even more than most of us, use all new information primarily to reinforce their existing worldview.

Irving is an archetypal heretic: certainly an eccentric, perhaps even a psychopath, and in any case just plain wrong. He deserved to be punished, we may feel. But people who question accepted truths are sometimes right (or just as right as the orthodoxy) – yet they too are likely to be outcast.

This is nowhere more obvious than in science, even though the modern myth has it that science is the haven of fearless, original thinking. So it was that in 1982, Rupert Sheldrake in A New Science of Life proposed the notion of "morphic resonance": suggesting that all living creatures directly influence the form of others of similar kind who come afterwards. It sounds weird – but Sheldrake was known as an outstanding scientist and his idea is testable, which is the criterion of bona fide science. So New Scientist ran a competition to suggest ways of testing it. In contrast, Nature, edited then by the conservative John Maddox, declared that Sheldrake's book should be burnt.

Since then, among other things, Sheldrake has shown beyond what is normally considered to be reasonable doubt that telepathy is a fact, not least in humans and in dogs. But he is hounded by the Skeptics, a society formed in 1992 to root out unorthodox thinking everywhere; and they worship "the amazing" James Randi, a stage magician who set up the James Randi Educational Foundation (JREF) to protect us from "the true danger of uncritical thinking".

The Skeptics and Randi (at one point supported by Nature) strive tirelessly to debunk all trials that claim to demonstrate the value of everything from telepathy to homoeopathy. Randi has offered a $1 million prize to anyone who can demonstrate any non-orthodox claim – and, he says, no one has seriously taken him up. However, says Storr, those who have tried to deal with Randi tell a very different tale: that whenever someone does rise seriously to his challenge, they can't agree on the rules.

Most of us write off Irving as a dangerous nutter, while Randi is widely seen as the heroic defender of rationality and hence of truth. Yet they are two of a kind. Both are convinced that they are "evidence-led", but both defend their worldview to the death whatever the evidence may throw at them. So too, as Storr relates, do the fundamentalist Creationists who see Darwin as the anti-Christ – and the fundamentalist atheists who aim to shoot them. But then – there but for the grace of God go all of us. It's how our brains work.

The Heretics throws new and salutary light on all our conceits and beliefs. Very valuable, and a great read to boot, this is investigative journalism of the highest order.

Colin Tudge's new book 'Why Genes Are Not Selfish and People Are Nice' is published by Floris in March