ONCE UPON a time, when the Beatles were still in Liverpool and no one had heard of LSD, Marshall McLuhan was an easy man to like. He could explain everything from Homer to baseball with a single, breathtaking theory. The few who had read him possessed the key to history, politics, art, literature and contemporary society. But now it has all been lost.

Like too many other good things, McLuhan has become public property. His face is on half the coffee-tables in America, and any self-respecting house wife can speak authoritatively about the electronic media, and the global village, and the death of print. The ideas have been diluted, distilled and vulgarized. No longer is it possible to discover McLuhan; it requires a strenuous effort just to avoid him.

The man most responsible for the popularization of McLuhan is McLuhan himself, in part because his ideas are so flamboyant, but more because of his conscious striving to get the message across. McLuhan considers himself a prophet with a mission: to end our colossal and perilous ignorance of media.

Media, McLuhan insists, are nothing more than extensions of some human faculty. The book, for example, is an extension of the eye. The telephone is an extension of the ear. Each society utilizes a number of different media, but emphasizes some more than others. Naturally when one medium dominates the rest, the human faculty of which it is an extension becomes more important than all the others.

FOR several centuries after Gutenburg's invention of movable type, Western society was tyrannized by the printed word, the book, and therefore the eye. For the book-oriented man, the relevant question is not whether he sees beyond his nose, but whether he knows beyond what he sees. We have been trained by our books to look, not to listen or to feel. And seeing imposes a very different perspective from hearing or touching. The eye can only move in straight lines, taking in one word or one idea at a time. The railroad, like the eye, moves in a direct, unbroken path, and could only have been conceived in an eye-oriented society. The same is true for the assembly-line. And the Maginot line. And the reception line, and the linotype machine.

The book was responsible for all the mechanical labor-saving devices of the industrial revolution. Protestantism, the Enlightenment and suburbia all owe their creation to the book and the eye. But in spite of its once-impressive power, the book has been overwhelmed, in recent decades, by the electronic media: the telegraph, the radio, the computer, and especially television.

The television presents a mosaic of thousands of tiny dots. To "see" the image on the screen, the viewer must participate, he must use his entire nervous system, and not just his eyes and ears, to fill in the spaces between those little dots. A child raised on television has entirely different techniques of sense-perception from a book-age child, and these differences are producing the West's most significant revolution since Gutenberg.

McLuhan is annoyed by the book-oriented people who criticize the content or television. He argues that the significance of the medium derives not from what it says, but from how it says it. In McLuhan's words, the medium is the message. And the electronic message is turning everything upside down. When we relied on our eyes we needed straight lines. But the television mosaic has destroyed the line and replaced it with the pattern. All of our lines are doomed. The book-age line on the backs of women's stockings has already disappeared because of television. Yet until recently, even in the face of such total upheaval, we refused to study television as a medium, and had no understanding of its effects. McLuhan's latest book, The Medium is the Massage, is an attempt to boil down his theory (which has already been boiled far too often) and to provide the definitive skeleton key to media study.

A joint effort of McLuhan and an artist named Quentin Fiore, The Medium is the Massage is unlike anything ever published before. While everyone else continues to write obsolete eye-oriented books, McLuhan has put out the first electronic book. It does not progress in an orderly, sequential manner, developing from an introduction through the main argument and on to the conclusion. Like a television commercial, it is designed to make an impact rather than to tell a story, and because of the extraordinary visual skill with which it was compiled, it succeeds magnificently.

There is almost no text. McLuhan relies on aphorisms, unusual type faces, and impossible lay-outs to put his ideas across. Much like television, which the book strongly resembles in shape, it requires a great deal of participation. Passages printed upside down or inside-out force the reader to become involved. Many of the pictures at first seem incomprehensible. Only if the reader participates-to a degree he would never participate in a conventional book-is he likely to recognize, for example, the grotesque blow-up of a human foot which covers five full pages.

But The Medium is the Massage, for all its impact, doesn't come close to Understanding Media: The Extensions of Man, which McLuhan wrote in 1964. Understanding Media is still the best available statement on the relationship between man and media McLuhan provides an explanation for everything imaginable. If you are curious why Kennedy won the presidential election in 1960, then McLuhan can help you out. It was the television debates. Kennedy had the shaggy, low definition look that viewers demand. On television, Kennedy didn't look like a millionaire or a Catholic or a politician. His image seemed blurred and it was easy for the viewer to fill in the dots to suit his own taste, own taste.

Nixon, on the other hand, was a high definition figure, whose features were too stark and hard for television. It was too obvious to the viewer what sort of man Nixon was and this reduced the interest in intensive participation. But if the debates had been broadcast on radio-a medium which requires much less participation than television-Nixon would have won easily.

Or if you have ever wondered why men seldom make passes at girls who wear glasses, McLuhan again comes to the rescue. He knows why girls wear patterned or mesh stockings, why Germans make better nuclear physicists than Americans, why an American is repulsed when a foreigner carries on a conversation only three inches from the American's face, why there is such widespread distaste for the war in Vietnam, why some Arabs wear alarm clocks in their turbans to gain status.

Many of McLuhan's ideas are surely wrong. He admits frequently that even he doesn't believe all of the things he says. But his approach is legitimate. McLuhan is no pop sociologist using stolen theories and distorted facts to grind out best-sellers. He is no charlatan.

But he is a Catholic and this is visibly reflected in his writing.