Cities are conspicuous among settlements because of their bulk and pace: Venice, Paris, or New York. Each is distinctive, but all share a social structure that mixes systems (families, businesses, and schools), their members, and a public regulator. Cities alter this structure in ways specific to themselves: orchestras play music too elaborate for a quartet; city densities promote collaborations unachievable in simpler towns. Cities, Real and Ideal avers with von Bertalanffy, Parsons, Simmel, and Wirth that a theory of social structure is empirically testable and confirmed. It proposes a version of social justice appropriate to this structure, thereby updating Marx's claim that justice is realizable without the intervention of factors additional to society's material conditions.