Psychiatrists (doctors who ultimately control the treatment of the mentally ill) are taught that there are clear boundaries between madness and sanity. From their perspective, either you do or you do not have illnesses such as schizophrenia or manic depression. I doubt that many of the rest of us think this (remember that sign that used to be hung in offices, saying, 'You don't have to be mad to work here, but it helps'?)

We all know someone who has one or more symptoms of these illnesses without actually meeting the psychiatric definition. One example is David Icke, the former footballer (he was a reserve goalkeeper) and ex-TV sports presenter, whom I interviewed in 1996 for a television programme and whose career took a surprising turn in 1991, when he called a press conference to declare himself 'the son of the godhead'. Icke explained this to me as follows: 'When I see a human being, I see them symbolically as surrounded by an eggshell. The inside is the real them, the eggshell is the veneer of programmed self that is keeping the real self, the real me, trapped inside, that is desperate to get out and say, "Yes, this is me", the self that has enormous love, wisdom, knowledge. And it's only when the eggshell cracks, as it did for me in the early Nineties, and the eggshell falls to the ground, that the real self starts to manifest itself.'

Like the vast majority of Icke's ideas, this contained no delusions or other signs of schizophrenia. In several books he has contended that the world is becoming increasingly homogenised by corporate power.

You probably agree.

However, there comes a point in his argument that is delusional. 'Over the past five years I've been on a journey of discovering who controls the world and how it's done. We've been massively duped,' he says. 'The robot radicals on the left have got to accept that their heroes, like Marx, were actually front men for playing off one side against the other. The far right have to accept that some of their heroes, like Hitler, were front men for the same forces.'

Icke claims to have proven that there is a global conspiracy, a pyramid of power, with lower level people as 'gophers' and higher ones as 'super-gophers'. For example, 'Reagan and Thatcher were the super-gophers. I wouldn't even dream of trying to name the people at the very apex of the pyramid of power, because they aren't even in the public arena.'

While Icke might briefly have met the criteria for a diagnosis of schizophrenia in 1991, when his eggshell cracked, I am sure he would not fit them today. And the interesting thing is that, contrary to the claim that madness is an illness distinct from mental health, there are dozens of studies showing that there are people like Icke all over the place... people such as George Bush, who, like more than half of his compatriots, would agree with the statement: 'The Bible is the actual word of God and is to be taken literally, word for word.'

Next week: the normal madness