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.

Cool media are those that require high participation from users, due to their low definition (the receiver/user must fill in missing information).

Film, for example, is defined as a hot medium, since in the context of a dark movie theater, the viewer is completely captivated, and one primary sense—visual—is filled in high definition.

McLuhan says that conventional pronouncements fail in studying media because they focus on content, which blinds them to the psychic and social effects that define the medium's true significance.

McLuhan observes that any medium "amplifies or accelerates existing processes", introducing a "change of scale or pace or shape or pattern into human association, affairs, and action", which results in "psychic, and social consequences".

To demonstrate the flaws of the common belief that the message resides in how the medium is used (the content), McLuhan provides the example of mechanization, pointing out that regardless of the product (e.g., corn flakes or Cadillacs), the impact on workers and society is the same.

[6] As an example of "media experts" who follow this fundamentally flawed approach, McLuhan quotes a statement from "General" David Sarnoff (head of RCA), calling it the "voice of the current somnambulism".

Moreover, media and technology, for McLuhan, are not necessarily inherently "good" or "bad" but bring about great change in a society's way of life.

Awareness of the changes are what McLuhan seemed to consider most important, so that, in his estimation, the only sure disaster would be a society not perceiving a technology's effects on their world, especially the chasms and tensions between generations.

This is necessary to avoid the powerful ability of any medium to put the unwary into a "subliminal state of Narcissus trance", imposing "its own assumptions, bias, and values" on him.

[8] Instead, an historical example of the embrace of technological assumptions happened with the Western world, which, heavily influenced by literacy, took its principles of "uniform and continuous and sequential" for the actual meaning of "rational".

He believed that media have effects in that they continually shape and re-shape the ways in which individuals, societies, and cultures perceive and understand the world.

Based on his studies in New Criticism, McLuhan argued that technologies are to words as the surrounding culture is to a poem: the former derive their meaning from the context formed by the latter.

Some media, such as film, were "hot" - that is, they enhance one single sense, in this case vision, in such a manner that a person does not need to exert much effort in filling in the details of a movie image.

McLuhan describes the term "cool media" as emerging from jazz and popular music and, in this context, is used to mean "detached".

Like Eco, he too is ill at ease with this reductionist approach, summarizing its ramifications as follows:[18] The list of objections could be and has been lengthened indefinitely: confusing technology itself with its use of the media makes of the media an abstract, undifferentiated force and produces its image in an imaginary "public" for mass consumption; the magical naivete of supposed causalities turns the media into a catch-all and contagious "mana"; apocalyptic millenarianism invents the figure of a homo mass-mediaticus without ties to historical and social context, and so on.Furthermore, when Wired interviewed him in 1995, Debray stated that he views McLuhan "more as a poet than a historian, a master of intellectual collage rather than a systematic analyst.... McLuhan overemphasizes the technology behind cultural change at the expense of the usage that the messages and codes make of that technology.

[20] Additionally, Brian Winston’s Misunderstanding Media, published in 1986, chides McLuhan for what he sees as his technologically deterministic stances.

The main example is the passage from mechanization (processes fragmented into sequences, lineal connections) to electric speed (faster up to simultaneity, creative configuration, structure, total field).