Was Mill right? This, ultimately, is the question raised by the argument over whether centrists should debate with rightists whether diversity is a threat.

Mill thought that:

[Mankind’s] errors are corrigible. He is capable of rectifying his mistakes by discussion and experience. Not by experience alone. There must be discussion, to show how experience is to be interpreted. Wrong opinions and practices gradually yield to fact and argument: but facts and arguments, to produce any effect on the mind, must be brought before it.

This wasn't wholly wrong. If he were to return to us today, Mill would probably be impressed by our progress since his time on attitudes to (for example) religion and women’s rights: the latter weren’t achieved by terrorism alone. And even during my adult life, we’ve seen a decline in racism and homophobia.

But. But. But. There are big dangers with debating diversity. One lies in the mere exposure effect. By treating the claim that diversity threatens the west as a legitimate issue, we risk making it respectable. In this way, racists come out of the margins and into the mainstream. It’s for this reason that Richard Dawkins refuses to debate with creationists. Diversitydebate

Another problem is that very few of us are rational Bayesians. Instead, when confronted with contrary evidence, we double down on our priors. And we tend to be much more sceptical about opposing evidence than supportive evidence. Even the most rational debate can therefore lead not to rectifying mistakes but to attitude polarization. This was first pointed out (pdf) back in 1979 by Charles Lord, Lee Ross and Mark Lepper. They showed people mixed reports on the effects of the death penalty. They found that, after reading these reports people who supported capital punishment became stronger in their support, whilst opponents of the death penalty also became more dogmatic. I’m not sure that events since then have shown that their finding was an isolated instance. The "marketplace of ideas" does not always select for the best. 

Yes, some people can be persuaded by single debates. But these tend to be those who were open-minded to begin with, rather than partisans. 

All this would be true even if racists and near-racists were debating in good faith. Many, however, are not. You can’t debate with liars.  

In this context, I fear that centrists are committing an old error, a version of the Wykehamist fallacy. This is the tendency to believe that their interlocutors are, at bottom, good chaps who are amenable to reason because they went to the right schools (as, in fact, many actually did).

You don’t need to disprove ideas – assuming such a thing can be done – to defeat them. You can marginalize them by ignoring them instead.

In saying this, I’m not arguing for infringing freedoms of speech. Just because you don’t share a platform with someone or give them prominence in other ways does not mean you are denying them rights to free speech. I’m not invited to write columns for national newspapers or to address student unions, but my rights are not thereby infringed in any way. The same should be true for anti-immigrationists.

So far, all I’ve said rules out debating with anti-immigrationists as equals but is compatible with us arguing in blogs and newspaper columns that diversity and immigration are good things.

There are, however, two dangers with doing even this.  

One is that it might commit a framing error. Framing the immigration issue in terms of the question “is diversity a threat?” begs the question. It assumes that we collectively have a right to reduce diversity by excluding others from our society. But as Chris Bertram has so ably shown, this is not necessarily the case.

In fact, though, I have another problem here. It’s about opportunity cost. If we are debating diversity, we are not debating other things. This matters, because an under-rated facet of political power is the ability to decide what gets onto the agenda. The more we debate immigration, the less time we devote to debating the failings of contemporary capitalism. In this sense, doing so serves a reactionary function, by taking debate about radical progressive social change off the agenda. I’ll leave others to speculate upon whether, for centrists, this is a bug or a feature.