If you doubt this, consider the ease with which the nostrums of alternative medicine — acupuncture, homoeopathy, reflexology — have become “complementary” medicine, to be offered in conjunction with conventional treatments, despite being devoid of evidence for their effectiveness. Consider too the resilience of such notions as Jesus of Nazareth’s marriage to Mary Magdalene and his founding of the Merovingian dynasty; communication with the spirit world; and the fallacy of “Intelligent Design”. These are so resilient, indeed, as to be topical.

The speculative history of Jesus and its supposed suppression by the Roman Catholic Church is the premise of the best-selling novel The Da Vinci Code. According to a survey published this week, the book is the overwhelming choice of MPs for their holiday reading. You can try to cite extenuating circumstances: MPs have an instinct for populism, and heterodox fictional accounts of Christian origins do have a distinguished pedigree in George Moore and D. H. Lawrence.

But this is something else. The Da Vinci Code is a testament to the attraction of conspiracy theories driven not by historical evidence but by animosity towards allegedly monolithic institutions. It is popular history for those who find Michael Moore too nuanced.

Spiritualism has, on the other hand, suddenly and surprisingly found a serious chronicler. He is Peter Lamont, whose biography of the Victorian medium, Daniel Dunglas Home, The First Psychic, was published last week. This volume is respectful to its subject almost to the point of hagiography, in a field owing its very existence to admitted fraud. (The first mediums, Kate and Margaret Fox, of Buffalo, New York, produced mysterious rapping and knocks by trickery as a prank to frighten their mother, as Margaret confessed 40 years later in 1888.) In his lifetime, Home was credited among other gifts with the power of levitation: he was reported at one seance to have floated out of a third-floor window and returned by the same route. But his enduring memorial is to have been mercilessly satirised by Robert Browning in Mr Sludge, “The Medium”:

Now, don’t, sir! Don’t expose me! Just this once! This was the first and only time, I’ll swear, — Look at me, — see, I kneel, — the only time, I swear, I ever cheated.

Lamont’s book is less entertaining. On the only important question of Home’s life — were his feats genuine? — Lamont piles up artful caveats before concluding lamely that “we do not always know what is going on . . . perhaps we will never know for certain”.

A taste for the irrational is not scarce in public life, and it is a pity to feed it. Even conditional irrationalism — a studied agnosticism between science and pseudoscience — is liable to encourage the more thoroughgoing article. Nowhere is this better illustrated than in the rise of the “Intelligent Design” movement in response to Darwinism.

In some US states, school boards are debating whether to allow the teaching in biology classes of competing explanations of the origins of life. Ostensibly open-minded, and commended as such by President Bush, this policy is clearly inspired by the conviction that the natural world can be explained only by the intervention of a conscious agent. In short, it is Biblical Creationism in less obviously sectarian guise: the latent chaperoning the blatant. It dismisses evolution as “only a theory”, yet curiously fails to lodge the same objection in other disciplines. When mathematicians speak of set theory, no one suggests they be required to give equal time in class to those who dispute that the next number in the set “one, two, three” is “four”. Intelligent Design deserves summary dismissal, but probably will not get it.

A letterwriter to this newspaper last week exemplified the problem. Referring to Christian businessmen in Britain who sponsor schools that teach biology in accordance with the Book of Genesis, he declared: “Even supposing their attitude to evolution were wrongheaded, it hasn’t stopped either one from evolving into a millionaire.” I have an ominous sense that many MPs — including the Prime Minister, who has defended the teaching record of one such academy — would share the assumption that economic betterment is a surer test of educational policy than the cultivation of the life of the mind.

So I make my own recommendation to MPs for their recreational reading. Rudyard Kipling is an unfashionable writer, but he had prophetic gifts greater than any spirit medium. His short story The Village that Voted the Earth was Flat concerns a group of motorists caught in a speed trap (it was written in 1913!), who exact revenge on the local magistrate who also sits as their MP. They set up a bogus “Geoplanarian Society” to embarrass the man. It promptly votes, in the name of the village, to declare that the Earth is flat. The ensuing national ridicule is compounded when the genuine Flat Earth Society turns up to celebrate its “dear friends and sympathisers”.

The longer we indulge irrationalism in public life, the sooner will our own Flat Earthers, in modern dress, arrive to recognise our kinship.

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