It's sequence of the introduction to media as extension of human body. Marshall draw a map as how everything around us from relationship to others, the environment we are set in, public sectors are all turning to what was never before. It's not how important media are, but what role the media are playing. They are being the substitute of the previous organs we utilize and sometimes hold complaints of. Each kind of medium is the new tool. Once we thought human's evolution was pushed by the usage of tools, but now we see how the tools are not only changing the outside world but are having more and more influence on our physical or psychological own self. Our ways of perceiving the world is getting along with the new eyes we are using. From the old time's ditto devices, our lives are shifted from the village norm to the more mechanic-like copies. Those copies are more regularly habitualized, standardized. Once into the phase of mass media, we are detached of the meanings or interactions, especially when we are the public, which was only enabled to exist with the printing. Individuals are more important since they are offered the possibility to have the platform to state for themselves. Of course we love arts. "The art is the visualization of culture", which won't be possible without the solid substances it acquires. The electronic circuitry, however, leads us back to the village. Because all of those new media, they seek participation as necessity. Meaning and content becomes once again significant. It's as if reappeared, the ancient norm of living. Then the whole subject, environment and so on, are experiencing the result of a butterfly effect. Then the media forces human being to change other related public world. The education, the politics, the history, the language, the authorship… You won't realize until you are only thinking about the container of the information. How democracy but also aristocracy the media can be. It's the former when you value the information as helpful; It's the latter when information becomes fierce flood and savage beasts. Media robbed our childhood, which was only a evolutional invention in the 17s, The format of media are twisting...forwarding...decentralizing.... US. That's why the final question terminates at : who are you.
Above is the simple logic Marshall McLuhan is sharing in the black-and-white but pictorial mini-book.