I think it's the Marthter doing his trick of going "one step beyond" - there is the "Ostrich" theory of putting your head in the sand so no-one will see you extended to the idea of if you have a blanket over your head the bogey-man can't see you and therefore doesn't exist, so if you put a piece of blanket over his head he can't see you, so he thinks he can't exist - I think Angua in one of the books says that it works not for long, but for long enough.