I have only linked to this in passing before, but I refer to this essay frequently at the moment and recommend it wholeheartedly. So here it is again. And some quotes:
McLUHAN: Because all media, from the phonetic alphabet to the computer, are extensions of man that cause deep and lasting changes in him and transform his environment. Such an extension is an intensification, an amplification of an organ, sense or function, and whenever it takes place, the central nervous system appears to institute a self- protective numbing of the affected area, insulating and anesthetizing it from conscious awareness of what’s happening to it. It’s a process rather like that which occurs to the body under shock or stress conditions, or to the mind in line with the Freudian concept of repression. I call this peculiar form of self-hypnosis Narcissus narcosis, a syndrome whereby man remains as unaware of the psychic and social effects of his new technology as a fish of the water it swims in. As a result, precisely at the point where a new media- induced environment becomes all pervasive and transmogrifies our sensory balance, it also becomes invisible.
This problem is doubly acute today because man must, as a simple survival strategy, become aware of what is happening to him, despite the attendant pain of such comprehension. The fact that he has not done so in this age of electronics is what has made this also the age of anxiety, which in turn has been transformed into its Doppelgänger — the therapeutically reactive age of anomie and apathy. But despite our self-protective escape mechanisms, the total-field awareness engendered by electronic media is enabling us — indeed, compelling us — to grope toward a consciousness of the unconscious, toward a realization that technology is an extension of our own bodies. We live in the first age when change occurs sufficiently rapidly to make such pattern recognition possible for society at large. Until the present era, this awareness has always been reflected first by the artist, who has had the power — and courage — of the seer to read the language of the outer world and relate it to the inner world.
But most people, from truck drivers to the literary Brahmins, are still blissfully ignorant of what the media do to them; unaware that because of their pervasive effects on man, it is the medium itself that is the message, not the content, and unaware that the medium is also the massage — that, all puns aside, it literally works over and saturates and molds and transforms every sense ratio. The content or message of any particular medium has about as much importance as the stenciling on the casing of an atomic bomb. But the ability to perceive media-induced extensions of man, once the province of the artist, is now being expanded as the new environment of electric information makes possible a new degree of perception and critical awareness by nonartists.