Alexander Garvin, natty in bowtie and jacket, watched commuters hustle through the gray, sunken concrete plaza at Citigroup Center on Lexington Avenue. Across 53rd Street, in the fading afternoon light, more New Yorkers ducked into a faceless subway kiosk on the triangular patch of wind-swept sidewalk — ostensibly a second public plaza — that occupies the southeast corner. This is the city’s public realm, or part of it.

What passes for public space in many crowded neighborhoods often means some token gesture by a developer, built in exchange for the right to erect a taller skyscraper. Mr. Garvin, an architect, urban planner and veteran of five city administrations, going back to the era of Mayor John V. Lindsay (1966-73), has spent the better part of the last half-century thinking about these spaces.

“The public realm is what we own and control,” he told me the other day when we met to look around Midtown. More than just common property, he added, “the streets, squares, parks, infrastructure and public buildings make up the fundamental element in any community — the framework around which everything else grows.”

Or should grow.

Writing in The New York Times last week, Christopher B. Leinberger, a professor of urban planning, took note of “a profound structural shift” in America during the last decade or so, “a reversal of what took place in the 1950s.” Back then drivable suburbs boomed while center cities decayed. Now more and more people want to settle in “a walkable urban downtown.” The most expensive housing in the country, and not just New York City, is in “high-density, pedestrian-friendly neighborhoods,” he said.