sunlight, and enclosure (the presence of buildings and a diversity of surroundings). The fundamental rule of the neighborhood sidewalk also applies to the neighborhood park: "liveliness and variety attract more liveliness; deadness and monotony repel life." Jacobs admits that a well-designed park in a focal point of a lively neighborhood can be an enormous asset. But with so many worthy urban investments going unfunded, Jacobs warns against "frittering away money on parks, playgrounds and project land uses too large, too frequent, too perfunctory, too ill-located, and hence too dull or too inconvenient to be used." City neighborhoods Jacobs also criticizes orthodox urbanism for viewing the city neighborhood as a modular, insulated grouping of roughly 7,000 residents, the estimated number of persons to populate an elementary school and support a neighborhood market and community center. Jacobs instead argues that a feature of a great city is the mobility of residents and fluidity of use across diverse areas of varying size and character, not modular fragmentation. Jacobs' alternative is to define neighborhoods at three levels of geographic and political organization: city-level, district-level and street-level. The city of New York as a whole is itself a neighborhood. The key local government institutions operate at the city-level, as do many social and cultural institutions – from opera societies to public unions. At the opposite end of the scale, individual streets – such as Hudson Street in Greenwich Village – can also be characterized as neighborhoods. Street-level city neighborhoods, as argued elsewhere in the book, should aspire to have a sufficient frequency of commerce, general liveliness, use and interest so as to sustain public street life. Finally, the district of Greenwich Village is itself a neighborhood, with a shared functional identity and common fabric. The primary purpose of the district neighborhood is to intermediate between the needs of the street-level neighborhoods and the resource allocation and policy decisions made at the city-level. Jacobs estimates the maximum effective size of a city district to be 200,000 people and 1.5 square miles, but prefers a functional definition over a spatial definition: "big enough to fight city hall, but not so big that street neighborhoods are unable to draw district attention and to count." District boundaries are fluid and overlapping, but are sometimes defined by physical obstructions such as major roadways and landmarks. Jacobs ultimately defines neighborhood quality as a function of how well it can govern and