How should economists engage with a man who knows nothing about economics? Matthew Bishop says we should treat him as a "worthy interlocutor in a way that values his opinion". I'd caveat this.

We should make two distinctions here. One is between the man who genuinely wants to learn more, and the one who is loudly spouting nonsense for political reasons.

The former deserves all our help. Geoff Riley and Diane Coyle offer useful suggestions for him, but I fear that there aren't ways of supporting such people; the media generally fails atrociously.

The latter, however, deserves our derision and contempt - of the sort that Douglas Carswell got when he claimed that the sun rather than the moon causes tides*. Such people are polluting public debate, and generally committing several cognitive biases as well: overconfidence; the halo effect (the belief that a policy that's good in one aspect must be good in others); motivated reasoning; and the Dunning-Kruger effect too.

In this context, I'm not sure about Matthew's analogy:

If physicists laughed at everyone who attempted to comprehend what is happening at CERN, or linguists mocked every grammatical error made by friends practising their holiday Spanish, people would soon give up trying to participate out of exhaustion.

This would be a bad thing if people were genuinely trying to understand and get better. Many, though, are just closed-minded bigots. One of the most deplorable trends of our time is the rise of narcissistic loudmouths and the media's encouragement of them. Such people should be told: shut up you ignorant lout**.

Here, though, lies my second distinction: between what is practical and what is ethical. Just because a man deserves our contempt does not mean it is practical to give it him. Matthew is right to say that if we are to change his mind, we should meet him halfway - by taking whatever sliver of truth there is in his beliefs and using it to enlighten him. As Blaise Pascal said:

People are generally better persuaded by the reasons which they have themselves discovered than by those which have come into the mind of others.

There is, however, a danger here. In taking mistaken ideas seriously, we risk giving them more credence than they merit. And persuasion might be more practical if it exploits cognitive biases than if it tries to overtly oppose them. On both counts, we might face a trade-off between truth and effectiveness, which risks us forgetting Galileo's words: "e pur si muove".

The issue here is, of course wider and deeper than merely how economists engage with the public: I suspect the same issues arise in science and other areas of policy.

The underlying problem here is: how should we cope with the death of liberal optimism? Advocates of free speech such as John Stuart Mill believed that "wrong opinions and practices gradually yield to fact and argument". In a post-truth world of asymmetric Bayesianism where people have had enough of experts such confidence is unwarranted. Liberals and experts have so far failed to come to terms with this.

* This just shows that we should leave the solar system to stop foreign bodies meddling with our seas. Let's take back control of our tides.

** I try to do this: there are huge numbers of subjects - including a lot of economics - about which I know little: I try to keep quiet about them.