If city planners were rock stars, Jaime Lerner would be Mick Jagger—utterly confident and virtually indestructible. At 69, Lerner became a legend by turning a slummy Brazilian cow town called Curitiba into an urban utopia. As the United Nation's authority on sustainable cities, he's helped places as diverse as Caracas, San Juan, Havana, Shanghai and Seoul to become more livable. Earlier this month, he wowed a crowd that spilled into the hallways of the Academy of Science for the Urban Sustainability Forum.

Compared to Curitiba, Philadelphia is headed backwards. While Curitiba grew from a population of less than half a million in the '60s to nearly 1.8 million today, Philly lost half a million. As Curitiba grew into an industrial hub, Philadelphia lost its manufacturing base. Curitiba opened scores of health-care facilities, while Philly has slipped into public-health crisis. As a whole, they got richer, while we got poorer.

Philly's politicians like to blame the city's demise on the suburbs' rise. But for Lerner, who served several terms as Curitiba's mayor, faulting the burbs for the city's incompetence is utterly bogus.

"The city is not the problem, it's the solution," was the message Lerner brought to an audience of more than 600 who came to hear how he'd fix Philadelphia.

Every city needs a master plan for livability, said Lerner, that is grounded on three concepts: 1) mobility, 2) sustainability and 3) identity. Before you build anything—a park, a high-rise, whatever—figure out first how people are going to get there; second, do the economic and the ecological math to gauge its sustainability; and finally, choose designs that express the city's cultural identity.

In practical terms, use cars much less, recycle much more and create buildings people feel good about. Because if you concentrate on what makes a place livable, economic development will follow.

Lerner knows livability, and Curitiba is a jewel. (After seeing a couple hundred slides, I'm ready to book a flight.) Mobility is put first. The city has a bustling high-rise district like Philly's, but its boulevards are wide, its traffic is sparse, and you'll wait less than a minute for a bus. Oh, and its transit system operates in the black, without a government subsidy.

Curitiba is also sustainable. Seventy percent of its trash is recycled, and people regularly even trade their trash for food. In Philly, there's a new and similar food exchange (see "Waste Not, Want More" on page 16), but our program is failing.

Curitiba's sense of its own culture is ubiquitous. Lerner relies heavily on artists to create public parks and cultural centers. "Artists have a special skin," Lerner told the audience. "They can feel society earlier." In Philly, we pay lip service to the specialness of the creatives, but Lerner actually lets artists lead the bureaucrats.

Mobility, sustainability and identity. But these alone will not make Philadelphia livable: having a master plan will. When asked how to deal with the power of developers, Lerner said that "big developers can't profit if everyone knows the plan of the city."

Excuse me, please, but what plan? At the moment, the plan for America's "next great city" is little more than a jumble of archaic zoning laws that everyone hates. We've got a patchwork of fixes and exceptions, a groaning machine greased by campaign dollars. And a mayor and a city council who can't or won't do anything about it.

Right now, our Planning Commission has neither a mandate nor any power. Both its board chairman and executive director are "acting" appointments—impotent placeholders, awaiting the next mayor's election. Meanwhile, the Zoning Board of Adjustment regularly butchers the rules to suit well-heeled developers. What passes for planning is done on-the-fly by members of City Council, whose vision of the future extends about as far as their next election.

Philly needs a man (or woman) with a plan, and I'd love Jaime Lerner to spend some more time with us. We also need a mayor unencumbered by a retinue of developers. Otherwise, Philadelphia's renaissance will surely expire stillborn, as we slip further into the muck of corruption.