No human being can write fast enough, or long enough, or small enough† ( †"smaller and smaller without limit … you'd be trying to write on molecules, on atoms, on electrons") to list all members of an enumerably infinite set by writing out their names, one after another, in some notation. But humans can do something equally useful, in the case of certain enumerably infinite sets: They can give "explicit instructions for determining the nth member of the set", for arbitrary finite "n". Such instructions are to be given quite explicitly, in a form in which "they could be followed by a computing machine", or by a "human who is capable of carrying out only very elementary operations on symbols."