McLuhan’s theory of communication, laid out in part 1 of Understanding Media, is centered on four major concepts. First, there is the idea that any invention or technology is an extension of human sensory organs and constitutes a new medium of interaction with the environment. Second, it is argued that media, as extensions of man, have characteristics that mold people’s experience quite independently from their possible use or content. Third, McLuhan defines media as either “hot” or “cool” and points out that this major difference determines the characteristics of the psychic reality they help to create. Fourth, and finally, it is theorized that a shift in the media orientation of a society inevitably leads to major patterns of change, particularly striking when the shift is from a hot to a cool medium or vice versa.

While the theory encapsulated in the phrase “the medium is the message” is perhaps the most misunderstood of McLuhan’s contributions, the distinction between hot and cool media is perhaps equally confusing to some. That may be because it derives from the communications engineer’s concepts of information density and semantic redundancy, concepts unfamiliar to most readers of social critiques. The distinction McLuhan makes, however, is quite clear: “Hot media are low in participation and cool media are high in participation or completion by the audience.” In other words, a medium that provides the recipient with much precise, standardized information—such as, for example, a photograph—is “hot.” A medium that requires the user to do a considerable amount of “filling in” through inference and imagination, such as in the case of a cartoon, is definitely “cool.”

The distinction between hot and cool media and the hypothesis that a shift in orientation from one to the other is likely to create major social upheavals are central points of Understanding Media. In the conclusion of The Gutenberg Galaxy, in which McLuhan fully discusses the impact on Western civilization of the phonetic alphabet and printing—typically hot media—a question is raised: What kind of perception and judgment would characterize “the new electric age.” Understanding Media tries to provide an answer to that question by examining how the print-oriented, grammar-bound, linear-thinking individual constituting the prototype of Western man for the last five centuries is reacting to the bombardment of the senses brought about by electric and electronic media, the first of which was the telegraph and the most typical of which is television. Through a systematic analysis of the characteristics of the major media affecting the Western world, McLuhan points out how the electric/electronic media are strikingly different from those that preceded them. Thus, they herald the radical breakdown of all the social forms human beings take for granted but which are in fact correlates of the “Gutenberg way of perceiving.”

In McLuhan’s view, literacy, especially literacy based on the type of phonetic alphabet that made the printing press technologically feasible, had a variety of consequences. The artificial dissociation of sight, sound, and meaning into a linear sequence organized through the eyes led to a parallel dissociation of the individual from the web of social relations typical of preliterate, tribal societies. The use of standardized symbols to signify sounds and their arbitrary combination into units of meaning led to similarly arbitrary abstract concepts of time and space. The lineality of the printed page led to a particular type of belief in sequential causality and encouraged abstract representations of space.

The description of the characteristics of the “Gutenberg way of perceiving” is a rather gloomy one. The reason is that the overdevelopment of the visual sense brought about by phonetic literacy...