Tim says that Keynesianism doesn't work because 'there's just no way, given political reality' that governments can run large and sustained budget surpluses during booms.

Such a view is widely shared; it's one reason why euro area finance ministers want to impose fiscal discipline through constitutional rules, rather than rely upon states' discretion.

But it's not just Keynesianism that is ruled out by 'political reality'. As Philip Booth points out, political reality also makes lost causes of lots of policies beloved of classical liberals such as Tim. There's little hope of liberal policies on immigration, drugs or prostitution. I suspect a flat tax is impossible because of the irresistible demands - reasonable and not - for loopholes, favours and reliefs. And there's not much chance of massive public spending cuts, given the power and interest of bureaucrats to resist them; one of the paradoxes of libertarianism is that one of the research programmes it inspired - public choice - does much to show why libertarianism is doomed to fail as a political strategy.

I say all this not to sneer at Tim, but rather to raise a question about the nature of political change.

Part of me wants to say that a flaw in classical liberalism - regardless of its other merits or demerits - is that it places too much faith in the power of reason, as defined by its own (perhaps dim) lights. Just as some soft lefties think the world would be better if we were less greedy, so classical liberals give the impression that it would be so if only folk were more 'rational.' Both are utopian. And both can be contrasted to Marx's view that revolutions require not (just?) people to be nice and rational, but instead a powerful constituency with an interest in achieving change. Classical liberals have no such constituency.

But that's what half of me thinks. Another half remembers Richard Cockett's description of how libertarian think-tanks helped - over very many years - to shift the Overton window; within my lifetime, private ownership of utilities, for example, has gone from being unthinkable by the political class to taken for granted.

There is, then, a question here of the roles of reason, power and client groups in achieving genuine political change. This question gets overlooked amidst the tribalism of day-to-day debate and a managerialism which worries about focus groups and day-to-day polling. And for me, this is a pity.