They love horse meat in France, so why do we turn our noses up? You fill your electronic notebook with all kinds of exotic detail and all the while you are analysing the mystery. Why does one culture ban the eating of an animal that another culture regards as top nosh? What is it all about? You chew the end of your intergalactic pencil and then boing - your eyeballs come out on foot-long red stalks as inspiration strikes. It's all about control. This amusing species called Homo sapiens has only lately emerged from prehistoric savagery; and what the human race fears most is a return to that darkness and disorder. So over the millennia the human race evolved the idea of taboo, as a way of creating social cohesion. Things were deemed to be nefastus, haram, forbidden. Individually and collectively, people developed little electric fences in the mind, and by agreeing on what was taboo they defined themselves; they defined themselves in opposition to others; and they helped to create a crucial sense of identity. By controlling small aspects of behaviour, society helped to control the big ones. Taboo, you conclude, was important in the creation of order. With growing excitement you continue your researches, and you observe several characteristics of taboos. They may become detached from their original rationale - shellfish and pork are less likely to give you food poisoning these days - and yet still have great force. Taboos are psychologically powerful since they affect inescapable features of human experience: diet, reproduction, sexual behaviour, menstruation, defecation and so on. You note that taboos are often accompanied by a great deal of hypocrisy. Various sexual practices denounced as unthinkable turn out to be fairly common, and as for eating horses - well, you have to wonder whether the British were really as naive as the newspapers make out. Surely, you ask yourself, people must have thought there was something a bit dodgy about the meat in a fairground hamburger; and surely they must have wondered about the export of all those British ponies to foreign abattoirs. Where did Shergar go? But if they suspected they were eating horse, they didn't mention it. It was taboo. The most interesting feature of the taboo, you discover, is the way it mutates over time. Things that were acceptable can become outrageous - and vice versa. Smoking is out, and will never come back in. I have been reading Herzog, the 1964 novel by the Nobel prizewinner Saul Bellow, and was struck by the way he refers to "negroes" - a term that would almost certainly preclude him from being read aloud on Radio 4 today. Or take the issue of gay marriage. If the polls are correct, most people just can't understand what all the fuss is about, because the taboo on homosexuality has gone - and seems to have been transmuted into a much more virulent taboo against any kind of underage sex. With assumptions changing so fast, it is no wonder that some people feel angry and bewildered. They aren't bigots; they've just been marooned by the fluctuating tides of taboo. As you get back into your spaceship, you draw several conclusions for the paper on human taboo that you will present to your fellow Martian anthropologists. You decide that taboos vary from country to country because in a way it doesn't matter exactly what the prohibition is: what matters is the fact of the taboo; and the process of agreeing - and changing - the boundaries of acceptable behaviour is central to what makes a society. Where, you wonder with a final flourish, will taboo go next? If the British and the Americans already regard the eating of dogs, cats and horses as haram, how long before all animals are objects of dietary taboo? In 100 years" time, you wager, the British public will be paying for memorials for the legions of animals that have died to feed them, and the British prime minister will issue an apology to all oxen that have provided the roast beef of old England. Of course it will all be nonsense, and people will flout the taboo. But that is not the point. The important thing about taboo is that it should exist.