"Only connect," the writer E. M. Forster said famously — and modern scientists working with network structures are learning how right he was. Forster was talking about how to tell a good story, but it turns out that the same principles for creating richly interconnected structures do apply to making good cities, or other good designs. And what's all the more interesting (and important) is how bad we've gotten at this in recent years — and why that came to pass. Jane Jacobs, the great urbanist and economist, put these ideas to intelligent use in her observation of what made cities such evident crucibles of economic productivity. It was proximities, she said, and networks of proximity, that allowed people to exchange knowledge and creative activities. These "Jacobs spillovers" are a major subject of research in economic science today. Something similar seems to help explain why cities are such resource-efficient places too. Just as cities promote "knowledge spillovers," they seem to promote "resource spillovers" too, where the waste of one process (say, heat from an energy plant) can become the input of another (say, to heat a building). But these efficiencies can only happen if there is a pattern of proximity, and the possibility of inter-connection. Cities, Jacobs realized, are webs of connectivity, between people, activities, and places. And these webs were all rooted in the connective pattern of the public realm — the street and the sidewalk. Not only did this public realm allow the creation of networks of connection on the physical scale of human beings and their movements, it also allowed users to control the degree of connections, and to disconnect and reconnect where they needed to.