At the start of ‘The Emperor’s New Clothes’, the concluding essay of Through the Vanishing Point, McLuhan cites a passage from Esthétique du Mal by Wallace Stevens:

This is the thesis scrivened in delight,
The reverberating psalm, the right chorale.
One might have thought of sight, but who could think
Of what it sees, for all the ill it sees?
Speech found the ear, for all the evil sound,
But the dark italics it could not propound,
And out of what one sees and hears and out
Of what one feels, who could have thought to make
So many selves, so many sensuous worlds,
As if the air, the mid-day air, was swarming
With the metaphysical changes that occur.
Merely in living as and where we live.¹

These 1944 lines from Stevens may be considered a foreshadowing amplification of McLuhan’s claim thirty years later in his 1972 Take Today:

There are only two basic extreme forms of human organization. They have innumerable variants or “parti-colored” forms. The extreme forms are the (…) eye and ear… (22)

“Language itself” is the range of relational possibilities between the eye and ear (“what one sees and hears”) accounting for

So many selves, so many sensuous worlds,
As if the air, the mid-day air, was swarming
With the metaphysical changes that occur
Merely in living as and where we live.

So it is that we may observe (following McLuhan in Through the Vanishing Point, 191):

the individual as a montage of loosely assembled parts.

These are not spare parts or any sort of mechanical bits and pieces; they are stage parts — roles, perspectives, identities (“so many selves, so many sensuous worlds”) — that an individual “puts on”:

All the world’s a stage,
And all the men and women merely players;
They have their exits and their entrances,
And one man in his time plays many parts²

McLuhan’s suggestion, comparable to Stevens in Esthétique du Mal, is that the different possible relationships between the eye and ear (“what one sees and hears”) structure all the different “parts” we play: “the meaning of meaning is relationship” (Take Today, 3); “visual and acoustic space are always present in any human situation” (Global Village, 55). The study of human experience therefore requires focus on the manifold of such eye/ear relationships or ratios. For it is not only “the individual” who “plays many parts”: any family or society or language or culture, too, is just (just!) such a “a montage of loosely assembled parts”.  

McLuhan’s central question has to do with “the air, the mid-day air” (echoing Shakespeare’s “mid-way air“) in which such ‘loose assemblages’ are possible and actual: “the medium is the message”. For it is through such a medium, alone, that “their exits and their entrances” can take place. No part or assemblage can achieve such “exits and (…) entrances” on its own, let alone its transitions to other “parts”. Unlike Baron Münchhausen, who extricated himself and his horse from a mire by pulling himself up by his own pigtail, each of these requires ground on the basis of which, alone, such assemblages and “flips” or “metamorphoses” between assemblages may take place at all. Each must originate in, and eventually return to, what grounds them.

2500 years ago Anaximander put it like this:

Into that from which things take their rise they pass away once more, as is ordained; for they [must] make reparation and satisfaction to one another for the injustice according to the appointed time.³