All the world's a stage for visionary economist Jeffrey Sachs. His mission? To eradicate extreme poverty. His method? To harness the financial power of the wealthy for the benefit of the destitute. His mantra? Yes, we can—and must. His frequent-flier mileage? Astronomical. Amy Wilentz observes the great optimist at work in Mali and Liberia

Thirty thousand feet above the Sahara, the economist Jeffrey Sachs is talking with Mali's minister of agriculture, Tiemoko Sangare, about a fertilizer factory. "I just read a northern Kenyan study on this," Sachs is saying. Sangare nods politely. The minister's English may or may not be good, but one thing is almost certain: He has never seen a verbal phenomenon that can compare to Jeffrey Sachs. A stream of fascinating facts, economic analysis, and political assessment issues forth from Sachs upon demand and once undammed can be stopped only with difficulty. Sangare's eyebrows are up, and his head tilts quizzically to the side. Sachs goes on talking, the minister nodding as a form of punctuation to Sachs's peroration, his disquisition, on fertilizer.

Below us is a wasteland of sand with a rosy vein of road cutting through it, a single thread making its way across the desert from Bamako, the bustling capital of Mali, to Timbuktu, some 400 miles away. Sachs's wife, Sonia Ehr-lich Sachs, a pediatrician, sits with her nose in a book on philosophy. The private jet we're flying in is nearing Timbuktu, and the darker stain of the Niger Inland Delta comes into view. I am looking out, taking in the desert's vastness, the snaky, arterial flow of the river, the subtle changes in color, and the pattern of squares and the glint of tin below that are sure signs of human habitation.

Where I see color, light, beauty, and the wonder of human survival against all odds, Sachs sees something else: a political economy unfolding. "Look at that little community down there on the edge of the water," he says, turning for a moment from his talk with the minister. "It's incredibly fragile." He goes on, excited. "Look at their distance from any market! Unbelievable. If there is drought, malaria, or locusts—-disaster!" The agriculture minister takes a sip of tea and looks relieved. The flow of fertilizer has been diverted.

The village where we are headed, Toya, some 16 miles outside Timbuktu, is part of the Millennium Villages Project, backed by the United Nations. Sachs launched the program in Sauri, Kenya, in 2004 with a $5 million gift from a philanthropist and the grand goal of eradicating poverty in Africa by deploying private funds from the developed world. He then raised $100 million more—$50 million from financier George Soros and the rest from individual donors (one of whom has loaned Sachs the jet we're flying in). Like the 79 other Millennium Villages, Toya is to receive $1.75 million over five years, which will go toward seed and fertilizer. The money will also go to water pumps, schools, health clinics, lunch programs for students, and antimalarial mosquito nets for every villager, as well as to power production, connectivity (cell phones and the Internet), and basic business institutions like microfinancing and farmer cooperatives. It's an all-or-nothing effort to end extreme poverty—along the lines of the UN's Millennium Goals, established in 2000.