According to Mattern, the city-as-computer metaphor is based on the exclusionary ontology of decision trees. That grounding makes discrimination inevitable, as decision trees split issues to make an unequivocal decision rather than looking for common ground. Mattern’s proposition builds on—or, better, grafts onto—the image of actual trees. At a higher resolution, Mattern proposes the image of intermingled branching; a city should be configured through arboreal decision-making. Within the new grafting metaphor of cities as intermingled trees––in short, forests—Mattern plants two advantages. First, the grafting metaphor invokes the uncountable actors that intertwine in municipal decision-making processes. Second, with grafting, new trees begin as branches of existing trees. Hence the metaphor fosters an idea of care and interdependence between social and physical infrastructures.
Compactly written, A City Is Not a Computer fuses four articles, previously published in Places Journal, with two new chapters that synthesize all the texts into the proposition for an arboreal urbanism. The introduction extends on the title by discussing Christopher Alexander’s famous article “The City Is Not a Tree.” For Mattern, Alexander’s formal analogy between physical and social networks becomes an entry point to compare data structures and assess their embedded cultural bias. The first two chapters catalog the failures of the computer metaphor; in the first, she looks at computer interfaces, and in the second, she intellectually challenges computational metaphors. The following chapters turn to the arboreal city grafting metaphor. They open with Mattern’s most salient case study, the library, which digitization transformed from a book depository into a place for public knowledge-making. Becoming centers for people to meet and communicate, libraries turned into accelerators in which people can engage creatively in forms of bottom-up community building.