In May, for the first time in human history, the world’s population was more urban than rural. But how can we deliver quality of life to our urban environments? Here, five leading thinkers give their answers. First, the novelist Jonathan Raban looks at why security risks threaten the city’s traditional great promise: the chance of anonymity. Also taking part in the debate are urban strategist Charles Landry, architect Ole Scheeren, director of the Serpentine Gallery Julia Peyton-Jones and architecture professor William Menking.