McLuhan's insight was that a medium affects the society in which it plays a role not by the content delivered over the medium, but by the characteristics of the medium itself. McLuhan pointed to the light bulb as a clear demonstration of this concept. A light bulb does not have content in the way that a newspaper has articles or a television has programs, yet it is a medium that has a social effect; that is, a light bulb enables people to create spaces during nighttime that would otherwise be enveloped by darkness. He describes the light bulb as a medium without any content. McLuhan states that "a light bulb creates an environment by its mere presence." More controversially, he postulated that content had little effect on society — in other words, it did not matter if television broadcasts children's shows or violent programming, to illustrate one example — the effect of television on society would be identical. He noted that all media have characteristics that engage the viewer in different ways; for instance, a passage in a book could be reread at will, but a movie had to be screened again in its entirety to study any individual part of it.
The book is the source of the well-known phrase "The medium is the message". It was a leading indicator of the upheaval of local cultures by increasingly globalized values. The book greatly influenced academics, writers, and social theorists.