Philip Green doesn't get much public sympathy, but he deserves our thanks for doing one thing: he is reminding us that economists have wrongly neglected David Ricardo's theories of rent.

Mr G is fighting landlords to cut rents - as indeed are several other high street names. Which evokes Ricardo. He showed that profits can be squeezed not by worker militancy but by rising rents.

The basic idea here is straightforward. Imagine, said Ricardo, there were an abundance of fertile land. A landlord could not then charge farmers rent: the farmers would just move onto other land. As the economy and population grows, however, the best land becomes fully occupied so farmers must use less fertile land. As they do so, the owners of the best land can demand rent from their farmers. And as worse and worse land gets brought into cultivation so the difference in the output of the best and worst land increases, giving owners of the best land even higher rent. As Ricardo wrote:

By bringing successively land of a worse quality, or less favourably situated into cultivation, rent would rise on the land previously cultivated, and precisely in the same degree would profits fall; and if the smallness of profits do not check accumulation, there are hardly any limits to the rise of rent, and the fall of profit....In a progressive country...the landlord not only obtains a greater produce, but a larger share.

This isn't just true of farmland. It also applies in retailing. Owners of the best sites - those with the highest footfall and greatest accessibility - can charge shop-owners a rent up to the difference in revenues they get between the best and worst sites. As Tim says:

Even in busy London stations coffee sellers don't make much money because the landowner simply increases the rent. In other words, it's the landowners via rents who make money in prime locations, not the operators of businesses.

Retailers such as Mr Green have got the hump with this.

But it's not just retail space of which this is true. It's also true of housing. As Josh Ryan-Collins writes in his superb book, Why Can't You Afford a Home?, "desirable location (typically in large cities) is inherently limited." And Econ 101 tells us that if demand increases when supply is inelastic, we see big price rises. As Londoners get richer, everybody wants to live in Hampstead, with the result that only Russian oligarchs can afford to do so.

Historically, there has been a mitigation of this. In a lovely paper (pdf), Katharina Knoll, Moritz Schularick and Thomas Steger show that house prices were actually flat in many countries for years in the first half of the 20^th century. A big reason for this was the development of commuter train lines. The opening of the Metropolitan line, for example, allowed workers to travel easily from Neasden to central London. That reduced the monopoly which landlords in the city centre had, thereby squeezing their rents - just as cheap fertilisers would have squeezed farmland owners' rents in Ricardo's model.

This effect, however, was only one-off - at least it will be until we invent teleportation. And it has been replaced by another more powerful effect - financialization. Credit liberalization in the 1980s removed the constraint upon house prices imposed by current incomes by allowing people to borrow more. The upshot is that there has been a strong correlation between house prices and interest rates: as the latter have fallen, house prices have risen relative to earnings.

For this reason, as Ryan-Collins explains, high house prices are due in large part to financialization. This, he believes, means that there is a case for banking reform and a land value tax.

The point here, though, isn't just about policy. It's also about how economists approach their discipline. As Ryan-Collins writes:

Land and money are two of the most neglected concepts in economic theory. Land is immobile, irreproducible and appreciates in value over time due to collective investment - none of these features apply to capital goods. Yet modern economics and national accounts treat them as one and the same.

Herein lies one reason (of several!) why economists should study the history of economic thought. Doing so can show that classic economists knew things which modern ones have forgotten.