Amputation results in a change of the sense ratios by which people experience the world. Pre-electric man's visual sense was very heightened, but radio increased his audile sense, moving him in a direction back toward his pre-visual tribal state. The effect of the addition of sound to movies was "to diminish the role of mime, tactility, and kinesthesis." This change in the sense ratios can have effects that do not directly relate to senses as well. When writing resulted in a greater visual emphasis, one effect was the possibility of viewing the world with individualism and introspection. Because every technology is an extension of a human faculty, all of them result in a change of sense ratios, and there is no way to resist such a change.
The effects of this change of sense ratios differ depending on the existing sense ratio of a culture. Because television is a mix of the visual and audile-tactile, it pushes an audile-tactile culture like Europe in a more visual direction, and a visual culture like America in a more audile-tactile direction. Specifically, television has moved Europe into a more visual emphasis in packaging and dress, and it has moved America to a greater appreciation for the audile-tactile spoken languages, food, and "the plastic arts."
The reason closure is inevitable is because, in order to use a medium, it is necessary to embrace it. In other words, we become subservient to these media, changing the way we live and operate to match with them. For example, "an Indian is the servo-mechanism of his canoe, as the cowboy of his horse or the executive of his clock."
Closure is part of a cyclical process. People are modified by technology, and, in turn, they discover new ways to modify their technology, and the cycle continues.
Because of numbness, media often do not mainly change the ratio of the sense to which they're most related, but to all the other senses. As examples, McLuhan asserts that "the effect of radio is visual, the effect of the photo is auditory."
The most obvious closure in response to a new technology is the demand for it. Obviously, a technology cannot be in demand before it exists; but response to technology goes beyond this, so that a new technology is automatically in demand. This demand can be explained by the fact that media are extensions of our senses. In the same way that we use every sense available to us unless we are deprived of it, we also use every medium available to us. This explains the tendency to leave the radio and TV on constantly.