Thomas Sutcliffe: The naked truth about bad TV

One of my fondest memories of Channel 4 dates from the year of its launch - a time when some Golden Agers would have you believe that the channel was a broadcasting hybrid of All Souls' high-table, a top-ranking comedy club, and the Royal Shakespeare Company in a good season. But the programme I'm thinking about would be unlikely to make it on to anyone's list of TV treasures.

It consisted, so far as I can remember it, of an airmail letter from a feminist collective in New Zealand, read aloud on air as the camera panned down the writing paper. And as I watched this utterly baffling transmission - and tried to work out what kind of communal cerebral haemorrhage had ushered it through the commissioning process - it dawned on me that it was a good candidate for Worst Programme I've Ever Seen, one of those elusive absolutes that language suggests should logically exist, but which usually prove very hard to pin down.

As soon as I realised that it was a candidate, it became fascinating, of course. Would the production team ruin the Platonic perfection of their atrocity by including any moving images? Or would they disqualify it from eligibility by ending after just a few minutes - revealing it to be just an early example of the scheduling Polyfilla with which Channel 4 still occasionally stops up listings chinks.

Again, if my memory is reliable, neither thing happened. It rolled on uninterrupted for something like 20 minutes - and ever since has provided me with a kind of laboratory benchmark for just how bad television can get. The only qualification to this thought being that it is quite possible that there are viewers out there who recalls it as a zenith of their viewing experience.

I was reminded of that programme while flicking through the Radio Times, which this week contains an article by John Naughton in which he offers his own Bottom 50 of British Television - a list that kicks off with Naked Jungle - the Five programme that offered the nation the chance to see Keith Chegwin in the raw - and concludes, somewhat arbitrarily, with Crossroads.

You will probably have spotted the conceptual problem already. Any list of terrible television that includes Crossroads has clearly fudged some issues. This isn't a matter of saying that any programme that runs for 24 unbroken years can't be all that bad. Television has some curious ecological zones that can sustain the unsustainable for astonishing lengths of time. Nor is it a case of saying that Crossroads falls into the extensive category of "so bad that it's good". It's more that truly terrible television wouldn't stir any recall at all.

By definition, it's a bit difficult to think of these programmes - and not easy artificially to provoke a memory, either - since they never make it into media surveys or archive highlights. You can find them by going through library copies of the Radio Times - but even then, you're very unlikely to clap a hand to your head and say, "How could I forget that!". Along with the vast bulk of television, they're like Mission: Impossible messages - once they've been transmitted, they self-destruct, leaving no trace behind them. If television came in a packet, they would feature on the ingredients list as bulk filler - no discernable flavour, no detectable nutritional value, but quite useful for making the box feel full.

And I would argue that, in this context, mediocre is far worse than bad. Because it's one of television's duties to be terrible. It is an inspection hatch through which we can look at the inner workings of our society - at its cardiac pulses and digestive churn. It isn't always a pretty spectacle, of course, and in some cases it can be downright nauseating, but even the most deplorable programmes (or, more accurately, above all, the most deplorable programmes) will teach you something.

There's no rule that says symptoms have to be pleasant. And if you need an intellectual alibi to watch terrible TV, try this: imagine yourself watching the same programme in 100 years' time - when its crassness will have petrified into hard historical evidence and the tedium will have burned away. I wonder myself what future historian's will treasure - the programmes whose excellence we now take for granted? Or those that provoke a sense of shame as we watch them?

Which isn't to say that there's no point in distinguishing between good and bad TV - I don't want to put myself out of a job - only that bad programmes are an inevitable part of the ecosystem, and one with a contribution to make to the medium. It's best to avoid them if you can - but worth recognising their accidental qualities when you can't.