The medium is the message

The clearest case, McLuhan said, is the electric light. It carries no words, no pictures, no story. So hardly anyone thinks of it as a medium at all. He called it "a medium without a message", in the everyday sense that it has no content, and said it is noticed as a medium only when someone uses it to spell out an advertisement or a name.

Yet it changed daily life enormously. Electric light turned night into usable time, for evening work, study, shopping and sport, and let surgeons operate at any hour. McLuhan's point was that the light does its work "whether the light is being used for brain surgery or night baseball". Neither could happen as it does without it, which is exactly why the light itself, not any one use of it, is what reshaped how people live.

If a medium's content is not its message, what is that content? McLuhan's answer was that the content of any medium "is always another medium". Speech is the content of writing, writing is the content of print, and print was the content of the telegraph. The content of a film, he added, is a novel, a play or an opera.

This nesting explains why content is easy to notice and the medium easy to miss. McLuhan compared content to "the juicy piece of meat carried by the burglar to distract the watchdog of the mind". While we follow the story, the medium works on us unseen, so the content distracts us from the medium itself. The usual view, that all that matters is how a medium is used, he dismissed as a kind of numbness.

McLuhan used "medium" far more broadly than newspapers and television. The subtitle of Understanding Media is The Extensions of Man: for him any technology is a medium, because each one is an extension of some part of us. In a later book he gave examples. The wheel extends the foot, the book extends the eye, clothing extends the skin, and electric circuitry extends the nervous system.

An extension does more than add power. In McLuhan's view each new one shifts the balance among the senses, and people adjust to it without noticing. He found a picture of this in the Greek myth of Narcissus, the youth who mistook his own reflection in the water for another person. McLuhan linked the name to narcosis, meaning numbness: we fail to recognise our extensions as parts of ourselves, and so we never examine them.

McLuhan also sorted media by how much they leave for us to do. A hot medium extends one sense in "high definition", which he defined as "the state of being well filled with data". A photograph is hot; a cartoon, a few lines on a page, is cool. Hot media leave little to fill in, so they are low in participation. Cool media give less and leave more to fill in, so the audience must take part.

His paired examples make the idea concrete. Radio and the telephone both reach only the ear, but a phone call, he said, gives the ear only a meagre amount of information, so radio is hot and the telephone cool. On a screen, film is hot and television cool. A lecture is hotter than a seminar, and a book hotter than a conversation.

McLuhan's history of media begins before writing. Where people have only speech, words last only while they are spoken, so everything a community knows must be kept in memory and passed on aloud. His former student Walter Ong, in Orality and Literacy (1982), described how such oral cultures shape knowledge to be remembered, in rhythm, repetition and set phrases.

Writing changed the sense that words travel through. McLuhan said the phonetic alphabet gave its users "an eye for an ear": words became silent marks to be looked at, read alone, away from any speaker. He argued that this habit trained people to think of themselves as separate individuals rather than members of a listening group. Like the railway's, the alphabet's deepest effect, on his account, had nothing to do with what anyone wrote.

The Qur'an shows two media carrying one text side by side. Its name is related to the Arabic word for recitation, and it was first taught aloud: the Prophet recited each revelation, and his Companions learned it by heart and recited it back to be corrected. Scribes also wrote the verses down, and after his death the caliph Abū Bakr had them gathered into one written collection.

Zayd ibn Thābit, who led the gathering, knew the Qur'an by heart, yet he took a verse only when writing backed it as well as memory, so each medium checked the other. Here writing did not turn the words into silent marks read alone: the written text is still learned aloud, from a teacher. Many Muslims memorise the whole Qur'an, and the written copy, the muṣḥaf, is checked against that recitation.

Two years before Understanding Media, McLuhan's The Gutenberg Galaxy (1962) asked what printing did to Europe. Johannes Gutenberg's press, at work by the 1450s, made identical copies fast and cheaply. Economic historians estimate that Western Europe produced almost five million handwritten books in the whole fifteenth century, but more than twelve million printed ones between 1454 and 1500 alone, and over two hundred million in the next century.

McLuhan cared less about which books were printed than about the reader print made. Rows of identical type, read silently from left to right, trained what he called typographic man: private, individual and linear in thought. He tied print to the rise of individualism and of nationalism. The historian Elizabeth Eisenstein, in The Printing Press as an Agent of Change (1979), argued that print did transform learning by fixing and spreading texts, while faulting McLuhan's lack of evidence.

In The Gutenberg Galaxy McLuhan divided history into four ages, each opened by a new medium: oral, manuscript, print and electric. Print, he thought, spread information slowly outward from centres to margins. Electric media reversed that. The telegraph, then radio and television, moved news at the speed of electricity, so distant events reached everyone at once.

McLuhan's name for the result, a phrase he made famous in that book, was the global village: a whole planet drawn into the condition of a village, where everyone hears the news together and feels involved in it. In Understanding Media he called it "the new world of the global village". He did not promise harmony. In a 1977 interview he said the global village "is a place of very arduous interfaces and very abrasive situations".

Neil Postman carried McLuhan's question into the television age. In 1971 he founded a graduate programme at New York University in media ecology, the study of media as environments that shape how people live and think. In Amusing Ourselves to Death (1985) he revised the slogan to "the medium is the metaphor": each medium favours some kinds of talk and makes others hard.

Postman argued that print had suited long, connected arguments, while television turns news, politics and even religion into entertainment. One trend fits his worry. Daniel Hallin measured it in network news coverage of US presidential campaigns: the average sound bite from candidates and other prominent figures fell from 48.9 seconds in 1968 to 8.9 in 1988, and ordinary voters' from 13.6 to 4.2. Hallin doubted technology alone explained it, pointing also to Vietnam, Watergate, image-managed campaigns and fiercer competition for viewers.

McLuhan died in 1980, before the web, yet his question is still asked of new media, and the answers differ. In The Shallows (2010), a finalist for the Pulitzer Prize, the writer Nicholas Carr built on McLuhan to argue that the internet, with its links, alerts and constant switching, trains people to skim rather than read deeply, whatever sites they visit.

The psychologist Steven Pinker argued in The New York Times that same month that new media have always caused moral panics: the printing press, newspapers, paperbacks and television were each once denounced as threats to people's minds. He argued that experience does not rebuild the brain's basic capacities for processing information, and that the answer to distraction is self-control, as with any other temptation. The disagreement shows McLuhan's legacy fairly: he taught people to ask what a medium does to us, not to take the answer on trust.

McLuhan's critics pressed two main objections. The first is technological determinism, the view that technology drives social change by itself. The cultural critic Raymond Williams, in Television: Technology and Cultural Form (1974), argued that people develop technologies for purposes they already have, and that who controls a medium, such as a broadcaster, shapes its effects. If a medium's effect were the same whoever used it, he wrote, we could "forget ordinary political and cultural argument and let the technology run itself".

The second objection is vagueness. McLuhan wrote in bold aphorisms rather than testable claims, and his categories slip. Television was cool partly because its 1960s picture was coarse, so it is hard to place now that screens are high definition. Even so, interest in his work revived from the early 1990s, and his questions are still put to each new medium.