(FORTUNE Magazine) – Garrett Hardin is still not exactly a household name, but over the past two decades he has developed something akin to a cult following. Now professor emeritus of human ecology at the University of California at Santa Barbara, he has a reputation, mostly deserved, as a deep and original thinker. On the dust jacket of his latest book, Filters Against Folly (Viking, $15.95), you can read an extraordinary testimonial from superinvestor Warren Buffett: ''Page for page, I learn more from reading Hardin than I do from reading any other author.'' Most of Hardin's writings have dealt with ecological issues, but somewhere along the way he has expanded his charter and emerged as a kind of specialist in thinking. His main objective in Filters Against Folly is, quite simply, to tell you how to think. If the book has a message, it's that a wide range of people should be able to think clearly about issues that have traditionally been the domain of experts. Hardin is contemptuous of experts, a sentiment reflected in the book's subtitle, How to Survive Despite Economists, Ecologists, and the Merely Eloquent. Clear thinking, Hardin tells us, requires tools. The tools he has in mind are what he calls filters -- techniques making it possible to screen out the nonsense endlessly coming at us. The book is about three of them: the literate filter, the numerate filter, and the ''ecolate'' filter. We need a literate , filter, says Hardin, to help us recognize rhetorical devices that make people feel good but get in the way of straight thinking (for example, the statement that life has ''infinite value''). However, the chapter on literacy is a bit perfunctory; most of the book is about those other filters and the ways in which they help people to be numerate and ecolate. Numeracy, in Hardin's scheme, is an ability to think in terms of numbers, proportions, and rates. He is good on the dangers of innumeracy, and criticizes some ecologists who have ''an almost wholly literate orientation.'' He cites their broad approval of the words of Francis Thompson, the turn-of- the-century poet: ''Thou canst not stir a flower/Without troubling of a star.'' Bringing things instantly back to reality, Hardin observes that the nearest star, Alpha Centauri, is 25 million million miles away, and that all the physical influences we know about, such as gravity, magnetism, and light, have their effects diminished by the square of the distance involved. Stern conclusion: ''The poet's assertion is numerate nonsense.'' In developing the case against innumeracy, Hardin heaps scorn on the famous Delaney Amendment -- the federal regulation that bans from food any chemical found to cause any amount of cancer in any species at any dosage level. He points out that too much of any substance, including oxygen, is lethal, and concludes that the Delaney Amendment is ''a monument to innumerate thought.'' Of ecologists who would apply comparable logic to environmental issues and ban all pollutants outright, Hardin writes acidly: ''With friends like these, the environment needs no enemies.'' Being ecolate, Hardin says, means to ask: ''And then what?'' To concede that a particular policy will have a specified result is not to settle the issue; the next question is what else will happen. You can assume that something else will happen because, in one of Hardin's best-known aphorisms: ''We can never do merely one thing.''

Among the disasters created by non-ecolate thinkers was Egypt's Aswan High Dam, completed in 1970 and intended to produce electricity and increase the amount of year-round irrigation. Not foreseen were numerous undesirable effects. By keeping irrigation channels perpetually wet, the dam favored the survival of snails and parasites associated with a debilitating disease called schistosomiasis. The dam also stopped the yearly buildup of silt in the Nile delta, a process that had created farmland and offset erosion by the sea. Available farmland is now declining. ''In probably less than 100 years,'' says Hardin, ''5,000 years of successful agriculture will be brought to an end.'' AS AN ECONOMIST, I was intrigued to note that many of the failures identified by Hardin with non-ecolate thinking are also failures to understand basic economic principles. This shouldn't really be surprising, since economics, like ecology, is supposed to help us understand the unintended consequences of behavior. Hardin himself realizes this, and cites Adam Smith's insight that participants in a market help each other, even though that is not their intent, simply by pursuing their own self-interest. One powerful example of economic and ecological insights reinforcing each other is provided by the so-called tragedy of the commons, which Hardin has written about in the past and returns to in Filters Against Folly. He reminds us that a photograph from space, published years ago in the weekly Science, raised a fascinating question. It showed a 400-square-mile lush green pentagon in the middle of the barren Sahel desert in Africa. The contrast, stark and puzzling, was explained by an agronomist who visited the area. He found a privately owned ranch. A barbed-wire fence protected it from the animals grazing the surrounding land to death. The tragedy of the commons comes in many different versions, Hardin believes. He points to numerous other instances in which the environment is degraded because nobody owns the land or air or water, and so nobody has much incentive to prevent pollution. Taking the argument a step further, he notes that subsidies are a disguised form of ''commonization'': the greater the subsidy, the less the nominal owner of an asset really has at stake. On his analysis, we have unhealthy commonization when municipalities build grandiose airports after discovering that 90% of the cost will be paid by the federal government. As you might suppose, Hardin is rough on businessmen and bankers who like to commonize costs while privatizing profits (which is another way of saying that they're being subsidized). Harsh judgment: ''Typically, they say they are defending and conserving private enterprise. A medieval highwayman could have claimed as much.'' In arguing against policies that he identifies with commonization, Hardin keeps telling us that needs do not create rights. His examples repeatedly remind us of follies related to subsidies and bailouts, and they will % generally cause libertarian thinkers to feel a warm glow of agreement. As the argument progresses, however, you see that Hardin is not at all a consistent defender of individualist values. His discussion of medical economics leads you down a curious byway. He argues that the most dramatic accomplishments of modern medicine -- liver and heart transplants, for example -- are happening because the costs have been commonized. Taxpayers are bearing the burden for care of the aged and those with kidney diseases; in other cases, insurance companies have in effect commonized the costs among their policyholders. ODDLY ENOUGH, however, Hardin doesn't want to privatize the commons in these cases. Instead he appears to favor some quite arbitrary limit on the age of the patients and the kinds of defects that would be eligible to receive such heroic treatments. This implies that he would actually consider barring older people from some kinds of coverage, even if an insurance company offered it. While criticizing what he sarcastically calls the ''medical commons,'' Hardin turns out to believe in the ultimate medical commons: he would allocate medical resources as though they were in some large pot and available to be ladled out by the government. Perhaps not surprisingly, I found myself often talking back to Hardin when he was discussing his differences with economists. He accuses us of ignoring the fact that many businessmen over the years have endangered their employees' health by having them work in unsafe conditions. But economists have not ignored this fact: they have instead explained it. Adam Smith noted that workers who were exposed to extra risks commanded higher pay. Modern labor economists have shown that the pay premiums associated with risky jobs are often very high. Hardin seems curiously oblivious here to his own rule: you can never do merely one thing. If owners were merely ordering workers to tackle risky jobs, and not paying attractive premiums, they would find themselves short of workers. HARDIN ALSO HAS a curious beef about discounting. Economists discount the future, using interest rates to make it comparable with the present. Hardin seems to feel that this is somehow foolish, and he offers an argument that he seems to regard as a clincher: he notes that a hardwood forest, which takes 100 years or more to grow, would not be worth investing in because the ultimate payoff is worth so little today. (At a 10% rate of discount, $1 million in 100 years is worth only $73 now.) - What's curious about this argument is that Hardin nowhere shows such calculations to be incorrect or irrational. Instead he simply pronounces unacceptable the view that we shouldn't invest in hardwood forests; he rejects economic rationality in favor of ''a higher form of rationality.'' But where does this higher form come from? And how does this rejection of discounting differ from the various non-numerate arguments he freely derides all through Filters Against Folly? For a man who positions himself as an unremitting skeptic and questioner of orthodoxy, Hardin seems curiously unwilling to give reasons for some of his own pet assumptions. He assumes, for example, that slowing population growth would necessarily raise per capita incomes. But that is a controversial proposition among economists, and it's not hard to point to many parts of the world -- Hong Kong, for example -- where incomes and population have soared together. As all of the above suggests, Hardin is a difficult man to categorize. It's hard to predict his views in one area from those in another area, and while this opens him to the charge of inconsistency, it is also part of his charm. He is often original and just about always fun to read. Considering the heavyweight themes he's tackling, he surely deserves a lot of points for that.