Content is media.
One main argument of the work is the relativistic argument that content is merely another medium being expressed or commented upon by another medium. One would expect that the effect of a piece of art would be its designed effect, but actually, McLuhan states that even the content of a piece of art is merely the iteration of another medium within the formal confines of the first medium.
Content is ineffective.
Therefore, content does not yield effects on the public, but rather, the public's experience of the medium carries the meaning. For instance, when the first televisions entered homes, it was the experience of television that mattered, not the programming or the meaning in the shows.
Meaning is in the form of the medium.
The meaning of an experience (according to McLuhan) is found in the person's experiencing the form of a medium, so that what happens is that regardless of what a movie is about, for instance, the experience will always be the same: "I saw a movie." That's the real meaningful effect of the film, not the message the film was hoping to communicate. This idea was highly controversial by the way, and understandably so. It is rather ironic.
Even a light bulb is a medium.
This view of media is highly theoretical, and McLuhan admits this, even going so far as to say that a light bulb is a medium, because it creates an environmental effect through its form. This would be an expression of a medium without content.
The irony of sociology.
Ironically, by participating in life, each person subjects themselves to the homogenizing effects of media, since we're all shaped by our experience of the every day world. This means that sociology does not depend on what types of interests or tastes a person may have, but rather, which media they participate in. Everyone who watches television has something in common according to McLuhan.
