A few years ago, I served on a jury. When the prosecution finished its case, I thought, "this evidence is pathetically weak: I must acquit the guy." Then the accused took the stand. His testimony was so wildly implausible that I inferred that he was guilty. This taught me a lesson - that people can be not just lousy advocates for their own positions, but actually counter-productive ones.

This has always partly the case. "Never believe anything until it has been officially denied" is a quote attributed variously to Otto von Bismarck, Claud Cockburn and Yes, Minister. But it seems to me to be more true now than ever before.

I said yesterday that Blairites undermined their own case by wibbling about "electability". But it's not just they who are counter-advocates. So are right-libertarians: you'd never guess from their hypocritical shilling for the rich that there is in fact a good argument for free markets. Feminists' narcissistic obsession with trivia seems to me to subtract from the case for gender equality. Drivel about the "nation's credit card" makes me marvel that any sentient being can be a Conservative. And the aggressive sexism and terrorist apologists on the far left remind me of Marx's saying: "All I know is that I am not a Marxist."

One reason why things have come to this sad pass is that a lot of writing is not even trying to be an exercise in persuasion. Sometimes instead it is the revelation of character - the character often being that of a thundering twat. At other times it is mere preaching to the choir, which, as Cass Sunstein has pointed out, leads to group polarization in which each sect becomes even more convinced of its own self-righteousness.

All of this is to endorse Robin Hansen:

There just is no "debate"; there are just different sides who separately market their points of view. Just as in ordinary marketing, where firms usually pitch their products without mentioning competing products, intellectuals marketing of points of view also usually ignore competing points of view.

To which I can only add: not just "intellectuals".

We can, however, lean against this tendency by a process of imaginative reconstruction. If I want to know the case for Blairism or Conservatism or free markets, I don't merely read Blairites, Conservatives or free marketeers. Instead, I ask: if I were they, what sort of arguments would I use? I tried to do this yesterday, and here and here too.

Proper political debate has, to a large extent, vanished. But perhaps we can reconstruct it.