The city is the most complex form of social organization. Perhaps this explains its temporal endurance and global diffusion. It antedates the beginnings of the world's great monotheistic religions, and has outlived the empires of antiquity to which its origins can be traced. From the perspective of urban time, therefore, the beginning of the third millennium of our era does not appear to have any particular significance. Indeed, if we are to speak of cities in terms of millennia, we had better adjust our chronology. In Western terms, the first urban millennium probably began around 800 BC and ended with the collapse of Roman civilization in the third century AD. The second millennium began around 800 AD and ended around 1800. The third urban millennium started around 1800 at the conjuncture of three revolutions: the industrial revolution which harnessed technological innovation and scientific inquiry to more productive uses of energy and new uses of materials, the political revolution which enshrined individual rights and democratic process in law, and the demographic revolution which pushed back the average age of death and increased the size of the population. The third urban millennium is already two hundred years old.
From another perspective, however, the transition into the 21st century does represent a major turning point. Within a few years, more than half of the world's population will live in cities. This makes learning how to manage space better an urgent priority. Our ability to plan wisely for this future is grounded in some two hundred years of human experimentation and experience. The record has not been uniformly positive. Urban growth has often accompanied immiseration for millions of people; class tensions in industrial cities were a factor in the diffusion of two of the most sinister ideologies of the 20th century, communism and fascism, both of which discredited planning; and the environment is still not easily cleansed of the wastes and pollution which accompanied urban growth in the past. How can we manage space better?
There is a paradox about planning. Urban and regional planning is an exercise that commits resources for decades. Yet the circumstances and factors that will affect life in cities, even just 10 or 20 years from now, are almost impossible to predict. There is no alternative to planning in the context of uncertainty, but many of the policies and procedures that have shaped decision-