The idea, Ron, is that the economist won't bother to even look to see if the dollar is there, because he assumes that even if there was a dollar on the sidewalk, someone would have picked it up already.

There are flaws beyond number with this example--first being that in America, the economist is not going to be walking down the sidewalk, he is going to be driving, or, if he is in New York, riding in a Town Car, and the meter is going to be running, so if he was to stop to pick up a dollar that he saw on the sidewalk, it would actually cost him money.

Furthermore, as everyone knows, that money on the sidewalk belongs to some homeless man, because these squares of pavement are his territory, and anybody else that touches it is in big trouble.