They Met the Man Who Wasn't There

Yesterday, upon the stair,
I met a man who wasn't there.
       Antigonish, by Hughes Mearns (1899)

     You go to the city centre, or to the shopping complex, and you see hundreds of anonymous strangers passing by. You assume they are normal, flesh-and-blood people going about their normal, flesh-and-blood activities, just like you. Indeed, it would be a very strange world if it were not so. But it is a very strange world. So how do you know that every last one of those anonymous strangers is really, physically present?