From a letter to Milena, quoted in Speaking Into The Air: A History of the Idea of Communication, by John Durham Peters:
Written kisses don’t reach their destination, rather they are drunk on their way by the ghosts. It is on this ample nourishment that they multiply so enormously. Humanity sees this and fights against it and in order to eliminate as far as possible the ghostly element between people and to create natural communication, the peace of souls, it has invented the railway, the motor car, the aeroplane. But it’s no longer any help, these are evidently inventions made at the moment of crashing. The opposing side is so much calmer and stronger; after the postal service it has invented the telegraph, the telephone, the wireless. The spirits won’t starve, but we will perish.
Haven’t read much of Speaking Into The Air yet, but it looks interesting.