What do you get from a political party? A cocktail of top-notch beliefs that you can go right ahead and believe.

The main pleasure to be had from your prêt-a-croyer values is to clap your hands repeatedly each time someone at a podium repeats one. I know this, having recently attended A New Hope – a conference on the future of the Labour party. Admittedly, the conference organisers - Compass - are independent, but the mood was very much that of a party's membership yearning for group identity, asserting itself like a thousand half-blown vuvuzela horns.

When the Athenians invented democracy, they didn't bake into it a requirement for political parties. Their model consisted of speeches for and against a position, followed by a show of hands. They may have had long-running divisions over certain issues, but there was no concept of fixed and opposing belief systems. They would surely have considered the anti-debate tribalism that resulted from loyalty to a dominant party as deeply undemocratic.

A party is like a religion; it does your thinking. If you adhere to its beliefs, you're moral. If not, you're not. Simple. It's a mistake to assume that a belief that resides in an adult head carries some innate value - some legitimacy by virtue of its existence. Most beliefs are only there out of a process of obedience. They do not imply the believer has made a personal journey to authenticate them, to make them "his own".

Most believers don't like this notion. Even non-believers don't like it. "Pascal's Wager" says you should believe in God because it just makes sense to; atheists attack this with the criticism that one cannot "choose" one's beliefs. But that's mistaken; their argument values belief - and belief-forming - too highly. Of course people choose to believe things, and end up believing them with emotional force. But... so what? It means only that the truth-value of most beliefs is negligible.

In religion as in party politics, what counts is conformity and uniformity. The purging of heresy. Back at the conference, this self-herding impulse was apparent during a debate between Labour leadership candidates. David Milliband (the favourite by virture of his spray-on Blairness and Obama-optimised chin incline) recklessly made a "off message" claim about the possible merits of academy schools. Ululations and gasps rippled across the audience - who otherwise clapped eagerly at any mention of "radical change", "sustainability", or... whatever is on the permitted list. It was the Boo/Hooray Theory in action.

Granted, the Compass conference echoed with various appeals to "plurality". It was after all supposed to be about "ideas, campaigns and coalitions needed to create a progressive consensus". But there's a clear tension between a desire to ally disparate elements, and a need for group-identity or easy-chant slogans. Plurality seems at odds with the base impulse to party-forming. There were stalls in the foyers for all manner of rebellion - but what can a campaigner against academy schools possibly have in common with an activist against Israel? The answer is simple. Againstness. It's like Marlon Brando in The Wild Ones:

Girl: What are you rebelling against?
JD: (Pause.) What have you got?

This is the second objection to parties: the obligation to join in with vilifying or negating the outsiders. As seen from the Left, everything outside of it is "in crisis", and all that's needed for an illusion of "coalition" or a "consensus" is a shared enemy. In this case, that inevitable old demon: the West. It's the glue that holds "progressive" politics together.