The 50p tax rate is probably getting more attention than such a minor measure deserves. But there's a point here that's been missed - that there's a parallel between higher taxes on the rich and immigration controls. Both policies, their critics allege, have similar effects: they reduce the supply of skilled workers and entrepreneurs.

Which poses the question: how can one support an immigration cap but oppose higher taxes on the rich, as many Tories do?

The question's acute because a tax rise has a marginal effect on labour supply - and might even have a positive one if income effects outweigh substitution effects - but immigration controls have a direct effect. A 50p tax might at the margin stop some people coming to the UK to start businesses.But immigration controls certainly do.

There's one answer to this puzzle that's obvious gibberish - that the case against higher taxes is a matter not just of economics but of freedom. However, stopping a man hiring whom he wants is probably as much a restriction of his freedom than taxing him more. And stopping someone working here is surely a greater restraint than higher taxes.

Instead, I suppose the argument is that exceptions to the immigration cap can be made for entrepreneurs and skilled staff. This is the Simon Cowell theory of the state - the idea that governments can act as talent-spotters.

This theory runs into problems.

First, it doesn't work now; there's agreement that current immigration controls are so tight as to deprive businesses of some key workers.

Secondly, a cap on poor and unskilled migrants would deprive us of high-quality talent simply because many of these - and their children - go on to become skilled workers and successful entrepreneurs. In the better parts of Leicester you can't throw a brick without hitting a rich man who arrived in England in just the clothes he was stood in. And if we'd excluded poor migrants in the past, we'd have not have had entrepreneurs such as Lew Grade, Isaac Wolfson or Charles Forte,not to mention more Indians than you can shake a stick at, and we'd not have household name firms such as Tesco and Marks & Spencer.

Let's, though, ignore these objections and assume - contrary to everything Hayek thought - that the state does have the knowledge to act like Simon Cowell. This only poses the question: if government is smart enough to spot talented migrants, isn't it also smart enough to do other things Conservatives oppose, such as intervene in or even control large parts of industry?

My point here is simple. It's quite coherent (though perhaps wrong) to support both tax rises and immigration controls - say because you doubt that tax rises have adverse effects or because you think that social cohesion trumps economic efficiency. And it's entirely consistent (though perhaps wrong) to oppose both, as proper libertarians do. What I have a problem with are those Tories who oppose tax rises whilst supporting immigration caps. Doing this requires some considerable mental gymnastics.