In opposing a mansion tax, David Cameron reminds me of Corrie's Tommy Duckworth. And I don't just mean that both are hapless oafs whose efforts at getting out of debt have backfired.

Instead, take a common argument against a mansion tax - that it would hurt asset-rich, cash-poor widows living in big houses.

To an economist, this is not a bug but a feature. In forcing (very few, but let that pass) widows out of big houses, a mansion tax would encourage a more efficient use of scarce housing by freeing up big houses for bigger families. To non-economists, this seems heartless.

What we have here is a clash between efficiency and sanctity. Efficiency requires the widow to sling her hook. But sanctity says people should be able to live in places where they have happy memories. Cameron seems to have chosen sanctity over efficiency.

Which brings me to Tommy Duckworth. His qualms about Tina acting as a surrogate mother for Gary and Issy are of the same form. Just as homes are more than economic assets, so too is Tina's womb.

Objections to immigration take the same form. Whereas economic efficiency argues for freer migration, sanctity doesn't. Countries, like wombs and houses, have value beyond their narrowly functional economic uses.

There are other examples. Opposition to prostitution and legalizing drugs and to having markets in transplant organs (pdf) are all based on the idea that some things are too valuable to be traded away.

The point here is that efficiency is not an incontestable value. It collides with others, the belief in the sanctity of some goods.

Economists tend to have a tin ear here - they just don't hear the arguments against efficiency.

Which raises a question. How might we argue for efficiency over sanctity. I'd suggest three lines:

1. In many cases, the clash between efficiency and sanctity is also one between liberty and sanctity. This is true for immigration and trade in organs and surrogacy, though not the mansion tax example.

2. The cost of having some sacrosanct assets which are beyond trade often falls upon third parties. For example, banning a surrogacy market would deprive Gary and Issy of their chance to have children; keeping widows in big houses deprives families of the chance to live in them. In this sense, the desire for sanctity is a cheap preference - it's borne by others. It should therefore be discounted.

3. Who decides what assets are sanctified? Consider this argument:

OK, let's assume that high taxes on top incomes are inefficient, in that they deter productive activity. This doesn't mean we should oppose them. Allowing people to have huge incomes is repugnant in itself, as it elevates individualistic selfishness, and undermines social solidarity. Equality is sacrosanct. It shouldn't be traded away for efficiency.

Very few people take this line. But why not? Why are wombs or homes or countries sacrosanct and not tradeable against efficiency, whereas equality is? Who decides what's sacrosanct and what's not, and on what grounds? Could it be that sanctity thus serves a reactionary function?

I don't say all this to take sides. I do so merely to show that values are contestable, but that a debate about them is possible.