People believe in extra-terrestrial visitors. People believe in Nessie, and Yeti, and Bigfoot. People believe in ghosts and fairies. So there are probably one or two people left who believe in mermaids. But if there are, they were not consulted in "Sounds of Sirens", a programme about mermaids.

It took the broad cultural overview. "Mermaids", began the presenter, Fiona Shaw, "have surfaced throughout history in all sorts of folklores, fairy tales and fiction". You knew at once where you were: the mermaid a recurring figment of the human imagination, a fantasy, a myth, a symbol - ripe for interpretation and analysis. And a group of folklorists, cultural historians and novelists were at hand to do the business. Mermaids "are emblematic of all kinds of things" one said. "The most slippery part of our iconography" said another.

So they proved. The programme followed a fluid course around ideas about femininity, song, hair, wetness, fishy smells, sexuality, secret wisdom, death, all kinds of yearnings and fears which these borderline creatures have excited and embodied. It was itself a rather seductive, dreamy sequence of opinions and descriptions, mixed with suitable snippets of music and literature. And, as often happens, myth-analysis turned gradually to a nostalgia for mythical ways of thinking. The mermaid is "a distillation of the imagination", an enduring, life-enchanting symbol which it would be a chase to lose. Rather, mermaid tales need retelling in contemporary ways. In all this, a word from someone who claimed to have actually met a mermaid would have given things an edge.

All right, the non-existence of mermaids is a certainty, (even though a sighting as recent as 1957 was mentioned). But notice how this certainty allows myth-talk to proceed with perfect leisure. It's easy to polish up your potent old legends, preserve them as enriching truths or the imagination, when there's no risk that anyone will spoil the fun by taking things literally.

But imagine if some mermaid-fundamentalist did appear on the scene. It would get embarrassing. Our modern mythologists would have to treat him as strangely deluded. No, no! You're not meant to believe it! But it would become clear that in fact they were deluding themselves. What they want is to keep the power of myths but without the superstitions that made them powerful - a siren song, if ever there was.