Next time you turn on the television, you may well be inadvertently peeking inside the mind of a young autistic boy called Tommy Westphall. This is not an exciting new feature of Virgin's digital package, but rather a surprisingly convincing internet legend derived from the 80s hospital soap opera St Elsewhere.

In the final scene of the final episode of St Elsewhere, it's implied that the whole series has been a fantasy, with St Eligius, the series' setting, existing only as a tiny model hospital in a snowglobe belonging to Tommy, the autistic son of Dr Donald Westphall. Disturbing, sure, for St Elsewhere's fans; but disturbing for the rest of us too when we realise that this narratory woodworm has infested dozens more American TV series going back to 1951.

St Elsewhere's Dr Turner, for example, was investigated for murder by Detectives Pembleton and Bayliss from Homicide: Life on the Street. Their colleague Detective Munch once questioned The Lone Gunmen from The X-Files. The Cigarette Smoking Man from The X-Files smokes Morleys, the same fictional brand as Spike from Buffy The Vampire Slayer. Buffy spun off Angel, in which one of the clients of the evil law firm Wolfram and Hart is Weyland-Yutani. A Weyland-Yutani spaceship was spotted in a hangar bay in Red Dwarf, and so was the Tardis. All these shows seem to take place in the same fictional universe - so if St Elsewhere was all a dream, then so is Doctor Who.

You can also catch out everything from Seinfeld to The Wire to Lost to Knight Rider, as catalogued on a website called Tommy Westphall's Mind. St Elsewhere turns out to be a sort of apocalyptic Kevin Bacon - connected to every TV series you can think of, and metaphysically undermining them all.

Of course, not everyone is convinced. For one thing, I can (and do) dream about Whiplash the Cowboy Monkey, but - thank God - that doesn't mean Whiplash doesn't really exist. Similarly, Tommy Westphall can dream about Homicide's Detective Pembleton, but that doesn't mean Pembleton doesn't really patrol the Baltimore we see on our screens. You can read several more objections here.

Back in 1988, of course, no one knew that television programs (apart from Star Trek) would ever be subject to such thorough and merciless scrutiny. But in the age of the internet, when television writers play a trick, they have to think about the consequences - because if they don't, somebody else certainly will. If that cramps their creativity, then so be it. Tommy Westphall is like HP Lovecraft's Cthulhu - you look into his eyes, and he destroys your faith in reality. Television can't take another monster like him.