Cities are typically seen as the engines of modern economic life. Cities are thus principally planned to optimize work and other practical, rational, preconceived objectives, and are designed accordingly, with even leisure space serving well-defined functions. But people do not only gather together in cities to meet their basic physiological needs; they also come to cities searching for love, esteem and self-actualization, and to experience the diversity of the world around them and to learn to understand it (Maslow 1943). Cities have a wide range of functions and they serve a wide range of aspirations (Mumford 1961, 1996). Wirth famously defined the urban condition as ‘a relatively large, dense and permanent settlement of socially heterogeneous individuals’ (Wirth 1996: 190-91). But this expression misses the heart of the matter: it is the interactions among these diverse individuals, their mixing, which really constitutes urbanity, and which gives city life its special character and possibility. Urbanism without a certain degree of cosmopolitanism is just a mass of completely unconnected, alienated strangers. It is in public open spaces that people are best able and most likely to engage with the social diversity gathered together in cities.