Embarking upon a biography of the brilliant and strange writer Willie Donaldson, I have been spending an unhealthy amount of time contemplating pornography. Donaldson, who died earlier in the year, was for much of his life in thrall to an intense pornographic ideal, finding it difficult to attain and impossible to sustain. His friend Julian Mitchell, recalling a trip to Paris in which, as teenagers, they had lost their virginities to prostitutes, wrote that "it wasn't the tart but the pornography that did him in, he used to say."
Few are as honest about their sex lives as Donaldson was, but only the most grievously addled old diddler would argue that regular exposure to porn has no effect on a person's ability to deal with everyday life. It fries the brain and knocks the libido off-kilter, simplifying - purifying, in a way - the complexities of desire and gender. But at the other end of the process, those producing dodgy films and videos have, ever since sex on screen could be beamed into every home, been earning fortunes.
So perhaps Tony Blair's old pal Silvio Berlusconi was on to something when earlier in the week his government passed a bill to tax the sex industry. Like smoking, porn is bad for the consumer and, like the oil business, a few firms make absurd profits, so why not let the taxman do what the censor cannot?
Italy's legislators have a good record for thinking the unthinkable - this year's decision by Turin to fine dog-owners €500 if they fail to exercise their animals three times a day was an excellent idea, as was Rome's outright ban of goldfish bowls. Their government's decision to raise a 20 per cent charge on all forms of pornography seems likely to quicken the interest of governments elsewhere.
The cash payoff for governments would be impressive. The "adult entertainment industry" is now thought to make £31.5bn globally and is a boom business, thanks to a happy combination of sex obsession and technological advance: this year's hot product is the hardcore porn video that can reach you through your mobile phone - the ultimate double-digital experience.
It is not difficult to imagine Gordon Brown becoming rather enthusiastic about a measure that would earn stacks of cash from porn merchants and self-abusers while sending out a stern moral message. For those concerned at the unfairness of taxing toplessness at the same rate as troilism, the Treasury could work out some kind of sliding scale based on activities, number of bodies and so on.
Ignored by government for too long, pornography could play its part in national life, offering new areas of social inclusion. Some blue-skies thinker in Westminster might investigate whether it is technologically possible to harness the great surge of porn-inspired jiggering and juddering to the national grid, offering a new source of clean, renewable energy. The latest Paris Hilton video could provide a whole weekend's power for the town of Saffron Walden.
To those who believe it is wrong for government to benefit from the exploitation of vulnerable consumers, there is an easy riposte: it is doing it already, with its cheerful promotion of casinos. A trickier problem will face tax inspectors of the Inland Revenue's porn division. Obviously the Sunday Sport will be taxable, but what about other newspapers which are every bit as sex-obsessed - the News of the World, for example, or the Daily Mail? Those "erotic thrillers" which Channel Five used to schedule around bed-time on Friday nights would be taxable, but where would Channel Four's earnest, wet-lipped documentaries about the hot new perversions stand? Will the hilarious sex scenes in Rome earn the BBC a tax bill?
For porn is not just finding its mucky way into mobiles. It is everywhere, influencing the way we look at the world. Skinflick tricks and values are there in television commercials, in mainstream films, in the concerned investigations of broadsheet newspapers and, of course, on reality TV shows. When one of Italy's adult entertainment veterans suggested that, with its new tax, his government had "picked a fight with sex", he was not going far enough. It had picked a fight with contemporary life.
The debate is not a local one, and it is liable to be around for a while. Recently Blanche Lincoln, a prominent Democrat in the US Senate, introduced a bill to impose a 25 per cent tax on the revenue of internet porn sites. It was time, she argued, to stand up and say that enough is enough.
There, precisely, is the argument against trying to control the booming sex industry through taxation. It is one thing to discourage people from smoking by making tobacco expensive; discouraging them from watching sex films through taxation is entirely different. It is censorship by stealth and, given the increasing dangers to free speech across the world, would be more dangerous than anything, however smutty, that people would like suppressed.
So the debates in America and Italy have been revealing in their way. Those who believe that the best way a society can moderate the tastes and behaviour of its citizens is by tax punishments will soon find some other opinion where the government should declare that enough is enough. It is no coincidence that the party supporting the tax on sex in Italy is the formerly neo-fascist National Alliance. Awkwardly, pornography and free expression are political bedfellows.