Julian Simon and Paul Ehrlich, economist and biologist, represent the two poles of thought on the planet's future. According to Simon, the outlook is rosy. Technology will outpace resource scarcity indefinitely, he thinks. Even explosive population growth is no problem because more people means more Einsteins and Mozarts: on balance their contributions will outweigh their needs. (Caricaturing Simon is difficult; like Ehrlich, he often does it to himself.)

Ehrlich, a noted scientist and author of many bestsellers beginning with "The Population Bomb" in 1968, sees doom around every corner. Even the good news that food production in the 1980s rose rather than plummeted as he predicted becomes bad news: "I underestimated how badly we'd keep on depleting our topsoil and ground water."

Ten years ago, these two gentlemen laid their respective convictions on the line in a $1,000 wager proposed by Simon. Any taker could choose any five natural resources, whose prices should rise if resource shortages are indeed a problem. Simon bet that the prices would fall. Ehrlich took the bet. Oddly, for a biologist, he chose five metals -- copper, chrome, nickel, tin and tungsten. This fall he mailed Simon a check. The prices of all of them had dropped.

The most obvious of Ehrlich's mistakes was to ignore the fact that commodity prices have been declining for decades. He clearly believed that a turning point was at hand. It wasn't. Of 55 renewable and nonrenewable resources charted each year in the "World Resources" report, for example, only three rose in price in the '80s. Also, 1980 was a peak year for commodity prices, a fact that is admittedly much easier to see in hindsight. But Ehrlich didn't need to be an economist to have seen how long the odds were against winning.

The wager's significance lies in the lessons being drawn from it. To my knowledge The Wall Street Journal's editorial board hasn't gotten to it yet, but you can imagine what they will say when they do. In fact, the bet sheds no light on the chasm dividing these two incompatible world views or on which man's beliefs are farther from the truth. The chief reason is that prices, Simon's indicator, don't now reflect stress and scarcity of most of the resources Ehrlich is concerned about.

In some cases that is because the resource isn't measured. Ehrlich might have made a safe bet that the supply of healthy air worldwide would decline, or the availability of potable water, but he couldn't have proved it. Or it may be because the resource never enters a commercial market. Fuel wood in developing countries is free in that sense, but even a casual tourist can testify to its rising scarcity in many parts of the world.

Some resources are not priced because they are still officially viewed as infinite, even though everyone knows better. The supply of natural systems for use as waste receptacles is an obvious example. In still other cases, economics does not yet provide the methodological tools for measuring value. No one, for example, can calculate the worth of a standing tropical forest or of biological diversity.

Setting aside the extremes to which both Simon and Ehrlich push their arguments, a rationalist would say that bridging the gap between them is merely a matter of what economists call "getting the prices right." Calculate the value of the services ecosystems provide, and all will be well. Progress is being made in that direction, but it is not, as it sounds, a straightforward exercise in mathematics. It requires changing gut-level understandings -- the kind that are shaped early and change slowly, the kind that differ profoundly between biologists and economists.

Consider the paradox of renewable and nonrenewable resources. The latter -- oil, minerals, metals -- are in fact inexhaustible, whereas renewable resources can be finite. As a nonrenewable resource becomes scarce, it gets expensive. Demand falls and substitutes and alternative technologies appear. No one will ever pump the last barrel of oil or mine the last ton of iron ore. But a species driven to extinction will not reappear, eroded topsoil cannot be replaced, and the Antarctic ozone hole will take 300 years to heal.

Such threshold effects are common in natural systems. Biologists see them every day, and most are skeptical that technology can repair the damage after an important natural threshold has been passed. Economists, on the other hand, generally expect all resources to behave like nonrenewable ones. They see less likelihood of irreversible change and are more confident that whatever happens, technology can find a solution. They see mankind as a unique species, operating independently on the planet. Biologists see us dependent on natural systems and entangled in their fate.

Simon's wager is still open. Reportedly the ante is now $20,000. A smart bettor might wait until the middle of the coming recession and take him up on it. I won't. But not because he's right that the planet's resources are infinite (palpable nonsense) or that environmental trends are nothing to worry about. If he's interested in betting on more meaningful measures than today's prices, though, I'd consider it.

One last word. Don't bet on Ehrlich either. There's much that needs changing, some of it urgently, but environmental disaster isn't imminent.