The 'Occupy' protest around St Paul's Cathedral in London raises interesting questions about the nature of property rights. When a few anti-capitalist activists started pitching their tents on the forecourt of the Cathedral, the first thought of the police was to step in and tell them to move on. But then the Dean of St Pauls, the Rev Dr Giles Fraser, told the police that it was the Cathedral's private property, that the Dean and Chapter approved of the right of people to protest, and that the police should clear off. Which they did.

Since then, however, with 180-odd tents pitched round it and more arriving by the day, the Cathedral has had to close some of its facilities and restrict access to visitors. A bit unfortunate if you happen to be on a once-in-a-lifetime trip to London and you cannot actually experience the magnificent interior of Wren's Masterpiece. And unfortunate if you live or, more probably, work around St Paul's and are inconvenienced with noise and congestion as a result of the protest.

There's also a public health consideration. There is no water or sanitation in the encampment, of course, and St Paul's is not known for its lavish shower and toilet facilities. Plus the fact that, like the years-long encampment on Parliament Square outside the House of Commons, the protest is, well, a bit of an eyesore.

It's a point noted by Milton Friedman nearly 50 years ago in his Capitalism and Freedom that, for all we liberals or libertarians believe in property rights, the ownership of property does not necessarily allow people to do anything they like with it. He asks, for example:

Does my having title to land, for example, and my freedom to use my property as I wish, permit me to deny to someone else the right to fly over my land in his airplane? Or does his right to use his airplane take precedence? Or does this depend on how high he flies? Or how much noise he makes? Does voluntary exchange require that he pay me for the privilege of flying over my land? Or that I must pay him to refrain from flying over it? Does my having title to land, for example, and my freedom to use my property as I wish, permit me to deny to someone else the right to fly over it?

Friedman's point is that property rights are not indisputable, but are a matter of convention, settled over long periods by public debate, court decisions, and laws. Dr Fraser and the protesters should no more expect to be able to do anything they like on the Cathedral's land than travellers can expect to be permitted to form a settlement at Dale Farm without the same planning permission that everyone else must apply for. The forecourt of St Paul's Cathedral is, in an important sense, as much public property as it is private.

I'm all in favour of people being allowed to protest – the last government's blanket ban on demonstrations within a kilometre of the House of Commons was an absolute outrage – but I'm not in favour of allowing people permanently to occupy public space, or even private space if it has adverse effects on the public. If we were looking for a new convention, the easiest might be a 24-hour rule: demonstrations on public land, or on private land that might impact the public, are OK as long as they pack up the next day. Then at least we, the public, might be able to reclaim what is supposed to be ours, instead of having to endure permanent eyesores around our main tourist attractions.