His chief significance was as a teacher and writer on these subjects, which greatly influenced architectural thinking. His book "Collage City" (with Fred Koetter) is his theoretical treatise that sets out various analyses of urban form in a number of existing cities known to be aesthetically successful, examining their actually existing urban structure as found, revealing it to be the end product of a ceaseless process of fragmentation, the collision/superimposition/contamination of many diverse ideas imposed on it by successive generations, each with its own idea. In architecture his thinking paralleled his ideas about the city: he was nostalgic for nineteenth-century eclecticism, advocating that architecture in the modern age should abandon its purist abstraction and allow itself to be influenced by influxes of historical references.