We have briefly looked at some of the problems involved in running a biggest city like, say, Melbourne, keeping the road and rail systems running, policing, providing food and housing, and so on. In another lecture, I'm going to deal with what we must now call the megalopolis - cities with populations of ten million or more. However, first I want to go back in history to when the population of cities could be numbered in the thousands rather than millions. One of the earliest theorists of the city was, of course, Plato, who created an ideal city in his text, The Republic. The population of this city would be around twenty-five to thirty thousand at most. Oddly enough, the same figures were chosen by Leonardo da Vinci for his ideal cities. Now, of these twenty-five to thirty thousand inhabitants only about five thousand would be citizens. A reason for this might be that it is the largest number that could be addressed publicly at one time and by one person, and makes a voting system much easier to manage. Also, perhaps the numbers are kept deliberately low because a large population would be harder to control, or because, in practical terms, fewer inhabitants are easier to feed from local supplies without having to depend on outside sources.