Several years ago I was walking by myself on Schermerhorn Street in Brooklyn, New York. At this point downtown Brooklyn was not the chaotic cloud of cranes, dust, and empty skyscraping glass that it is today. Up ahead, I saw a man sitting atop the metal armrest of a bench, one foot planted on the bench’s seat, the other on the concrete. As I approached, I could see that he was staring directly at me and was tapping repeatedly at his wrist, which he had held out for me to see. The thing I remember most distinctly about this encounter was that I was scared.