Jane Jacobs, who died last month in Toronto at the age of 89, was one of the most original thinkers of our time, and a warm, modest woman. The striking thing about Jane Jacobs's most important work, Death and Life of Great American Cities, published in 1961, was that all by herself, out of nowhere, she completely stood on end the whole field of city planning and urban design. She showed that a body of thought, more than a century old, with professorships and schools, professional journals and shelves of books, didn't understand what it was talking about. And since its ideas were widely accepted by people who implemented urban policy, they were causing immense damage to city life. All the experts were trying to make cities like small towns, or like works of art, built on abstractions. They prescribed open space, grass, low-density population, projects that took people, especially children, off the streets. These experts and idealistic foundations and political leaders who implemented their ideas, destroyed miles and miles of healthy cities across the country, throwing hundreds of thousands of people out of homes and communities they had built and loved, in order to create high-cost slums and underused urban facilities. Jane Jacobs looked at cities with open eyes, observing what worked and how people live. She created a whole new way of thinking about cities - a way which respected people's preferences and actual behavior instead of trying to design cities for the planners' dull ideas about how people ought to behave. JACOBS UNDERSTOOD that the great strength of cities comes from the large number and variety of people who live and work in them, and visit them. This means that social arrangements have to be based on the public relationships of strangers and near strangers. Furthermore, she taught, only these informal social arrangements can keep a city civilized. Police and laws can only play a back-up and foundational part; the flesh must come from people. "The public peace," she wrote, "is kept primarily by an intricate, almost unconscious, network of voluntary controls and standards among the people themselves, and enforced by the people themselves." She described how people in well-functioning parts of cities played their roles in the subtle dance of civilized street life, and she analyzed the economic and physical factors that improved or prevented the dance. She reported about the streets in the North End of Boston - an old, densely populated Italian section that had unslummed itself with no help from outside. Nevertheless, it was still considered a slum by professional planners because of its dense population and old buildings. But Jacobs wrote that the streets of this neighborhood were "probably as safe as any place on earth," with no street crime for a generation. "The ubiquitous principle," she wrote, "is the need of cities for a most intricate and close-grained diversity of uses that give each other constant mutual support, both economically and socially." SHE EMPHASIZED the point that "the sight of people [on a street] attracts still other peopleâ€¦ something that city planners and city architectural designers seem to find incomprehensible. They operate on the premise that city people seek the sight of emptiness, obvious order and quiet. Nothing could be less true. People's love of watching activity and other people is constantly evident in cities everywhere." Four prosaic factors were critical for enabling people to produce vital city neighborhoods, according to Jacobs. They "must serve more than one primary function; preferably more than two. These must ensure the presence of people who go outdoors on different schedules and are in the place for different purposes, but who are able to use many facilities in common." Second, "Most blocks must be short; streets and opportunities to turn corners must be frequent." Third, there must be a mingling of "buildings that vary in age and condition, including a good proportion of old ones." And, fourth, there must be a sufficiently dense concentration of people. SHE LATER wrote two other very original and powerful books about cities, The Economy of Cities and Cities and the Wealth of Nations, and then turned her observant eye and powerful mind to other subjects, including an appreciation of different styles of ethics, described in her book Systems of Survival: A Dialogue. I met Jane Jacobs only once, nearly 20 years ago. I sent her an idea I had written about Japan which urged considerations parallel to some of what she had proposed in her book The Question of Separatism about Quebec independence and she responded very sympathetically, so I asked whether I could visit her in Toronto, where she had moved because of her opposition to American policy in Vietnam. She was very welcoming when I took my admiration - not to say awe - in hand up to her brick house on Albany Avenue for a conversation of several hours. She was not at all troubled about our profound differences on foreign policy, and we had a wonderful morning talking about federalism and ethics - the subjects of two of her later books. The mind that revolutionized thinking about cities 20-odd years earlier was still curious and creative, and very open to exchange with others. There are only a few people like Jane Jacobs in a generation. It is sad to see her depart, even though she lived a long productive life, with a 52-year marriage and three children in a close family. The writer is the research director of the Institute for Zionist Strategies, and author of The REAL World Order: Zones of Peace/Zones of Turmoil (with Aaron Wildavsky).