Behold! human beings living in a underground den, which has a mouth open towards the light and reaching all along the den; here they have been from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them, being prevented by the chains from turning round their heads.
   Plato, The Republic, Book VII, trans. Benjamin Jowett. Project Gutenberg #1497

[paraphrase, not a quotation] McLuhan's line was that the medium is the message: the form of a medium reshapes how people think and live, more than whatever content it happens to carry. (after Marshall McLuhan, Understanding Media, 1964, ch. 1)

Put those two next to each other and you get an uncomfortable picture. The wall in Plato's cave is a medium. So is a screen. So is a chat window.