“Often think of the rapidity with which things pass by and disappear. . . . For substance is like a river in a continual flow, and the activities of things are in constant change, and the causes work in infinite varieties; and there is hardly anything which stands still.”
—The Meditations, by Marcus Aurelius1
1. Marcus Aurelius, The Meditations (The Internet Classics Archive, 2000), http://classics.mit.edu/Antoninus/meditations.mb.txt.
If you were capable of watching all of the automobiles driving on the roads within a geographic area, mapping sources and destinations, volumes of people and objects moved, and directions of transfer, you would be able to tell quite a lot about the activities of the people in ...