On a brisk fall day that has put color into the cheeks of the most die-hard Londoners, I visit the British Library and check out the first edition of the book that still generates such heated debate. Malthus's Essay on the Principle of Population looks like an eighth-grade science primer. From its strong, clear prose comes the voice of a humble parish priest who hoped, as much as anything, to be proved wrong.

"People who say Malthus is wrong usually haven't read him," says Tim Dyson, a professor of population studies at the London School of Economics. "He was not taking a view any different than what Adam Smith took in the first volume of The Wealth of Nations. No one in their right mind doubts the idea that populations have to live within their resource base. And that the capacity of society to increase resources from that base is ultimately limited."

Though his essays emphasized "positive checks" on population from famine, disease, and war, his "preventative checks" may have been more important. A growing workforce, Malthus explained, depresses wages, which tends to make people delay marriage until they can better support a family. Delaying marriage reduces fertility rates, creating an equally powerful check on populations. It has now been shown that this is the basic mechanism that regulated population growth in western Europe for some 300 years before the industrial revolution—a pretty good record for any social scientist, says Dyson.

Yet when Britain recently issued a new 20-pound note, it put Adam Smith on the back, not T. R. Malthus. He doesn't fit the ethos of the moment. We don't want to think about limits. But as we approach nine billion people on the planet, all clamoring for the same opportunities, the same lifestyles, the same hamburgers, we ignore them at our risk.

None of the great classical economists saw the industrial revolution coming, or the transformation of economies and agriculture that it would bring about. The cheap, readily available energy contained in coal—and later in other fossil fuels—unleashed the greatest increase in food, personal wealth, and people the world has ever seen, enabling Earth's population to increase sevenfold since Malthus's day. And yet hunger, famine, and malnutrition are with us still, just as Malthus said they would be.

"Years ago I was working with a Chinese demographer," Dyson says. "One day he pointed out to me the two Chinese characters above his office door that spelled the word 'population.' You had the character for a person and the character for an open mouth. It really struck me. Ultimately there has to be a balance between population and resources. And this notion that we can continue to grow forever, well it's ridiculous."

Perhaps somewhere deep in his crypt in Bath Abbey, Malthus is quietly wagging a bony finger and saying, "Told you so."