When city planners talk about community, it’s usually in terms of physical spaces.

So it comes as a surprise that in his latest book, “Open Moral Communities” (MIT Press, 2000), Professor of City and Regional Planning Seymour Mandelbaum has almost nothing to say about the physical realm.

In a brief talk at the Penn Bookstore May 5, Mandelbaum explained that his book examines the types of moral communities humans form and how they work to bind people together.

The book — which he described as “an extended reflective essay cultivating a communitarian sensibility” — is actually a collection of essays and articles written over a period of two decades. What they have in common is an emphasis on communities as collections of like-minded people and a preference for what he called “the field of open moral communities” — the many informal groups people join based on shared values.

In his talk, Mandelbaum made it clear that mere physical proximity does not a community make. “Sometimes, neighbors do not form communities,” he said, while, through the act of writing and finding a like-minded audience of readers, “I [can] have an extended community of millions spanning the world.”

And even people who claim to be part of a single community can disagree on its definition. Using the current debate over a new home for the Phillies as an example, Mandelbaum noted that both Mayor John Street and Chinatown residents opposed to the mayor’s preferred site defend their positions in terms of what is best for “Philadelphia.”

“The really hard issues [in defining a community] are those that come from the common words we share,” he said. Many of the conflicts that eventually make their way to the Supreme Court, such as the current case involving a gay man kicked out of the Boy Scouts, are also arguments over community — who belongs and who does not and what are the rights and obligations of those who do.

“Communities create members with rights and obligations,” he said, “but they also necessarily create strangers with different rights and obligations.”

For Mandelbaum, the key to defining community is the existence of a shared outlook and set of values. “Only those groups that create shared moral orders and sensibilities are communities,” he said.