Amusing Ourselves to Death

Hey everyone! I'm beginning to read Amusing Ourselves to Death and it is actually pretty difficult to read for me. Postman uses vocabulary that is hard to comprehend so I constantly have to look at the dictionary. I know if I had any kind of electronic devices on while I was reading, I would become distracted easily. As I read the first chapter, Postman made many interesting points that made me really think. For example, when he mentions that techniques like writing or a clock is not only an extension of man's power to bind time but also shapes the way we think and the content of our culture. I would have never thought something so simple as a clock could shape the way we think. Some of the things he has mentioned already in the first chapter, I would have never of thought about. He says that the invention of the eyeglasses suggests the belief that anatomy is destiny by putting the idea that our bodies and our minds are improvable. So many people uses eyeglasses on a daily basis and probably would never of thought of them in that way. Postman uses many examples so in the book that makes his idea easier to understand. However, I'm still confused when he says that a medium is a metaphor. Can anyone clear that up for me?