Sam Brittan wants the Lib Dems to become left-libertarian. This is like the suggestion that I marry Victoria Coren - a nice idea, but it won't happen. Lib Dems are vote-whores, and there are no votes in left-libertarianism.

Why not? It's not because left-libertarianism lacks merit. As I've said, it doesn't. To a large extent, it is just the political expression of the second theorem of welfare economics.

Instead, one reason is that there are strong cognitive biases which undermine support for both equality and liberty. Another reason is that, because left-libertarianism get no attention in the media, it loses from the mere exposure effect.

But there might be another reason why left-libertarianism is so unpopular. It lies in Jonathan Haidt's The Righteous Mind. He says that just as our tongue has taste receptors, so our moral mind has moral receptors. There are, he says, six of these: care/harm, fairness/cheating, loyalty/betrayal, authority/subversion, sanctity/degradation and liberty/oppression.

The problem with left-libertarianism is that whilst it appeals to the liberty receptor, it doesn't appeal much to the others. For example:

- Support for stringent inheritance taxes are thought to violate the sanctity of family life. Relations between father and son are considered more inviolable than those between employer and employee.

- Support for free migration is seen as disloyal to existing communities.

- An unconditional citizens basic income gives people something for nothing and so undermines norms of reciprocity and our instinctive hatred of the idea that people might take a free ride on our efforts.

- The opposition to paternalism doesn't appeal to the "care" receptor; left libertarianism is a harsh philosophy to the extent that it tells people to get on with their own lives.

- Left-libertarianism is an anti-authoritarian belief, as it it denies that the state has the competence or authority to do very much. This turns off the authority receptor.

In these ways, left-libertarianism is, to use Haidt's metaphor, like a restaurant which serves only sweeteners. It appeals to only a fraction of people's palates.

Now, I don't think this is a good argument against left-libertarian ideas. Liberty and equality are good receptors, and there's plenty to be said for the economic efficiency too. And anyway, there's no logical reason why the state must appeal to all our tastebuds. If you have a taste for loyalty, authority or sanctity, you can find them elsewhere in a free society. As Nozick said, a libertarian state is a framework in which you can pursue your own utopia.

All I'm saying is that there are massive obstacles to left-libertarianism becoming popular. But let's remember that what's popular is not necessarily what's right.