Apparently McCartney is far from happy with his new wife already. She's spending twice as much on shoes as his last wife.
This statement is rather misleading in that it implies to the reader that something has been knocked, but in fact you have only typed the word knock twice and possibly said it to yourself in your head. With this in mind, of course no one is there and you are in theory, talking to yourself.
This is very much much conjecture, the original matter is based on false evidence and thus no one could possibly know if a shoe being on another foot would infact change the matter of that which we do not know.