How much is the natural world really worth? If you're studying its rainforests, for example, should their value be estimated in terms of the wood that they contain? Or should it also include the indigenous animal and plant species which are known to contain useful pharmaceuticals - such as the lethal poison from the skin of an Ecuadorean frog, discovered recently, which could lead to a painkiller that may be stronger than morphine but without morphine's side-effects? Or, given that "carbon-trading" will soon be a common phrase, in which countries buy and sell permits to pump out carbon dioxide, should we put a value on rainforests based on their ability to absorb atmospheric carbon, and hence mitigate the greenhouse effect?
Increasingly, earth scientists are trying to put serious values on to what our environment provides us with - a 24-hour-a-day, seven-day-a-week service economy that never shuts down or takes a holiday.
The focus of this examination is mostly at US universities, including Cornell and Stanford. At the former, Professor David Pimentel of the ecology department last month released a study, performed with eight graduate students, making a "conservative" estimate of the value of that service economy.
"If all the planet's biota, all the plants and animals and micro-organisms, sent a bill for their 1997 services, the total would be $2.9 trillion [pounds 1.81 trillion, or pounds 1.81 thousand billion]." The US's share of that bill would, by their calculations, be $319bn. When you bear in mind that the US Gross Domestic Product (GDP) - which measures the value of goods and services produced in the country - was around $6.8 trillion, you can see that the ecosystem is a significant, yet unrewarded, part of any economy.
If "the ecosystem" were in fact a trading partner which charged - for air, water, animal breeding, seafood, hunting and so on - then the laws of supply and demand might rapidly put the US into deficit. "When you compare our spending to preserve biodiversity to the benefits we reap, we're really getting a bargain," commented Professor Pimentel, whose work was published in the December issue of the journal Bioscience.
In fact the value Pimentel's team put on the world's ecosystem looks quite tame compared with two others published in 1997: one put it at $33 trillion, and another, by Gretchen Daily and colleagues at the biological services department of Stanford University, at $20 trillion.
Their estimate is based on the "replacement value" of the biosystem, rather than its direct "service value". The question Daily poses is: "If you were setting up a happy life on the moon, you would have to think: what can perform these tasks - air purification, detoxification, preservation? And how much would it cost?"
Yet, she points out, production indices don't take any notice of those sorts of values. "The public is completely unaware of it."
The fact that scientists have to point out these contradictions is partly because the history of economics - reaching back before the invention of money, to the time of barter - has never assigned self-ownership to things deemed "inanimate", a definition which itself was often stretched bizarrely.
Hence, "inanimate" things can be exploited by whoever first claims ownership of them. It's an attitude that has been applied to everything from fossil fuels to live animals, and even (in the times of slavery) to fellow human beings.
The key element remains the same, though. It is an axiom of economics that anything which does not have to be paid for has no monetary value - even if its absence would cause the most horrendous problems. (Think of the oxygen in the air.)
Experiments to mimic Spaceship Earth have been done in a small way from time to time: the most memorable was in 1991, when the Biosphere II project in Arizona tried to see whether eight people could live in a self-sustaining ecosystem in an enclosed dome.
The results were so disastrous as to be laughable. Paul Ehrlich, also of Stanford, recalls: "It cost $200m to build an ecosystem 1hectare in area for eight people, and $1m a year to power it. They thought they had designed a self-sustaining system. In fact bacteria in the soil started producing carbon dioxide which was then absorbed by the concrete the building was made of. The oxygen level in the air fell from 21 per cent to 14 per cent - like going from sea level to an altitude of 17,000 feet (5,200 metres). The nitrogen oxides concentration increased by a factor of 75.
"It all went crazy, vertebrate animals died, and they had to pump in oxygen from outside. All these things are supplied easily by nature - but we don't know how to replace them."
Professor Geoffrey Heal, an economist from Columbia University, reckons that the pace of environmental wastage could enormously increase the cost of basic necessities - such as water - within our lifetimes.
"It's a reasonable forecast that by 2020 the price of water in some parts of the world will be at least half that of crude oil, and the investment in providing it will be comparable with that for providing electricity."
But natural processes will almost always be cheaper than manufactured ones. "New York City recently had a problem in which water it gathered from the nearby Catskills mountains was becoming polluted. A purification plant would have cost $4bn.
"It was cheaper to buy the land around the water source and prevent people from using pesticides in adjoining land, to stop pollutants entering the source." Soil bacteria could then clean the water naturally, he pointed out.
But such changes could require changes in the forms of ownership of land, possibly back towards that used in medieval Britain, where on each piece of land there might be separate rights to grow crops, fish, hunt, live and raise taxes. Each right could be owned by different people.
While it might cause social upheaval to introduce such changes in modern society, "it will probably be bigger if we don't do it," according to Professor Heal.
The biggest problem, though, is that many urban dwellers are too cut off from the source of their goods, said Professor Ehrlich. "In New York people think food materialises magically overnight in the supermarkets. They have to be made aware that they are fundamentally dependent on ecosystems."
It's either that - or wait until nature sends its own form of bailiff around to collect the unpaid debts.