as above, so below ~

At Camp Quest, the five-day "atheist summer camp" for children that ended on Friday, campers were challenged to prove that unicorns do not exist. It is to be hoped that the children did not spend too much time on a logical impossibility. It is much easier to prove that God cannot exist because He is a contradiction in terms. However, both God and the unicorn exist as ideas, and ideas, whether muddled or not, are real. The imagination of a child who was utterly unfamiliar with either God or the unicorn would be cruelly impoverished.

A clever child might argue that the unicorn could exist because it is no more absurd than the narwhal whale. The twisted tusk of the narwhal is what was supposed to grow from the head of the horse known as the unicorn. The centrepiece of a 15th-century Flemish mille-fleurs tapestry in the Victoria and Albert Museum is a unicorn, with a horn exactly like that: a narwhal tusk projects from its forehead, and a heavy tail with flukes, like a whale's, flourishes above its back. The background is studded with symmetrically placed flowering plants, plus the odd exotic game bird. I would give much to know what the tapestried picture means. Are all the featured creatures imaginary? Is the invented world of human fantasy here presented as superior to reality? Without knowing more about the idea of the unicorn, there is no way I can know what I am looking at.

A horse with a horn is not a contradiction in terms, unless I define a horse as a hornless creature. It makes no odds that a horse has no need of a horn. The narwhal does nothing with its "horn", which isn't even a secondary sexual characteristic because some females have them as well as males. The tusk is actually an overgrown incisor tooth. In theory, horses, too, could start growing a tooth into a tusk and then into a horn. Rhinoceroses have single horns made of modified hair; the mane of a horse could one day develop into a kind of horn. It hasn't happened – as far as we know – but no evolutionist should discount the possibility.

What cannot be decided is whether the designers of unicorn tapestries, and there are many such, thought the beasts existed somewhere on earth. Some cross-fertilisation of faith with imagination would have been needed to generate the collective energy that produced the tapestry sequences of the late 15th century. The most famous of these is the seven-piece sequence of La Chasse à la Licorne in the Cloisters Museum in New York, which was made in Brussels or Liège between 1495 and 1500. The seventh tapestry, La Licorne Captive – which shows the unicorn, bloody but serene, resting in a circular corral set in a field of a thousand flowers – has inspired thousands of needlework kits and millions of tea-towels, posters and wall plaques.

Humans have imagined unicorns since antiquity. The earliest natural histories describe a one-horned, hard-hoofed, horselike creature, which gradually finds its way into bestiaries as the unicorn. In the 12th-century Bestiaire of Philippe de Thaon, the unicorn is represented as a female; it is attracted by the perfume released when a virgin exposes her nipple, and falls asleep by her side. A Strasbourg tapestry in the museum at Basle shows a bare-breasted virgin with flowing hair and a unicorn in her lap, caressing its mane and horn. The gender of the unicorn remains mysterious. Pisanello's medal of Cecilia Gonzaga (1447) and Annibale Carracci's design for Domenichino's fresco of the Maiden and the Unicorn in the Palazzo Farnese (1604) both celebrate the power of maidenly modesty to subdue animal lust.

For centuries, Vikings trading in Europe sold narwhal tusk as unicorn horn. It was believed to possess the magic properties of neutralising poisons and curing melancholy, and fetched a price by weight higher than gold. There are unicorns in the English Bible of 1560, and in the Authorised Version of 1611 that replaced it, apparently because of a misconstruction of the Aramaic for "wild ox", which was remarkable for size and strength but would not be tamed to the plough. The Scottish royal coat of arms was upheld by two unicorns, of which one was imported to the British crest when James I ascended the throne in 1603.

The counterpoising of the lion with the unicorn is a feature of the second most famous tapestry sequence to survive: the six-piece allegory of La Dame à la Licorne in the Musée de Cluny. In three of the six panels, the lion and the unicorn function as bearers holding pennants, the lion on the lady's right, the unicorn on her left, just as for the royal coat of arms.

We cannot now read the story told in La Dame à la Licorne or La Chasse à la Licorne. We have lost the rules of those iconographic games and replaced them with fantasy embodiments that are far cruder and nastier than the snow-white horse with the horn, the goat's beard, the cloven hoofs and the lion's tail.