I used to work on the 19th floor of a building overlooking the Hudson River in upper Manhattan. I was often fascinated by the ever-shifting traffic patterns down below on the busy six-lane Henry Hudson Parkway. A rush-hour accident could bring three lanes of traffic to a halt when a knot of cars backed up behind police cars and ambulances. Unobstructed traffic going the other way would soon jam up, too, as drivers slowed their vehicles and craned their necks to see what had happened. The delays took longer to clear than to form, sometimes persisting an hour or more after an accident had been removed.

This memory of my traffic-observing days came back to me when I thought about how to explain the difference between a “group of individuals” and a “population.” I remembered drivers in their vehicles down below, each making decisions about how fast and how close to follow the car in front, looking for a quick exit and trying to catch a glimpse of torn metal or bodies. The sum of the eagerness, frustration, and curiosity of this group of commuters was more than a series of momentary glances or flashes of brake lights. Individual drivers' thoughts and acts, added together over time, turned into traffic delays that themselves created additional glances and brake lights and sometimes even new accidents. Individual cars passed through, but their movement created traffic patterns that endured.