THE sound of stable doors being slammed shut is echoing throughout the land. Following the murder of April Jones by Mark Bridger, there is a debate – if lurid articles all over the papers can be dignified with the description “debate” – about whether to ban child pornography on the internet.

This took me back decades to my moral philosophy class at Liverpool University and a maxim drummed into us then: “ought” implies “can”. In other words, don’t decide you ought to do something if you can’t do it.

How could a single legislature such as Britain go about policing a global ban, even if everyone here agreed to it?

It is claimed that Bridger was corrupted by viewing child pornography and this led him to murder a little girl. Such disgusting images might provoke others to murder. Therefore, the images should be banned. This takes me back to another meeting of the moral philosophy class when the subject of pornography in general was being discussed. People were quoting the 1959 Obscene Publications Act which defines pornography as something that “tends to deprave and corrupt”.

Some students – you might say being students – insisted that pornography should not be banned because people had the right to become depraved and corrupted if that’s what they wished. Others were outraged by this libertarian view. I remember that everyone sneered at Mary Whitehouse and her campaign “to clean up TV”.

I remember also that I was shouted down when I said that it wasn’t a question of whether pornography has a tendency to deprave and corrupt, but that pornography is in itself corruption and depravity. Nobody managed to ban it in those days and no one will be able to ban it today.

Sexual scenes involving children portrayed for the perverted gratification of others is vile and especially so because the children themselves are powerless to prevent it.

Hideous experience of human nature tells us that, even so, there are people who want to see such scenes.

But even if everyone agreed to ban child pornography, how would we go about it? The task would require eternal vigilance and the ability to peer into every nook and cranny of the nearly infinite reach of the internet. It would be like trying to ban swearing.

But even if it were physically possible to ban child pornography, the attempt would soon run into legal difficulties and problems of definition.

You start with the good intention of wanting to do away with the palpable evil of child pornography and you suddenly find that, though you’ve failed, you’ve accidentally managed to ban family holiday snaps and videos of the infants’ nativity play. Don’t scoff. A few years ago a well-liked TV presenter was questioned about photographs she had taken of her own children in the bath.

This is the absurdity into which we descend when we start banning particular items. We fail to get rid of something bad but in the process we get rid of something good.

Moronically, many have imagined that banning guns and knives would prevent massacres such as that at the Dunblane school in 1996 and routine street murders. The fact is that if you ban guns and knives only the baddies will retain them. Do we ban the breadknife then? And if you can’t kill someone with a knife or a gun, you could always try bleach – or a nice soft pillow. Tell you what, let’s ban bans, eh?