[The imperative need today is] to understand the techniques and functions of the traditional arts as the essential type of all human communication. (McLuhan to Innis, 1951)

Joyce (…) saw that the change of our time (…) was occurring as a result of the shift from superimposed myth¹ to awareness of the character of the creative process itself. (…) The very process of human communication, Joyce saw, would afford the natural base² for all the future operations and strategies of culture. (Notes on the Media as Art Forms 1954)

Mallarmé (…) saw, like Joyce, that the basic forms of communication — whether speech, writing, print, press, telegraph, or photography — necessarily were fashioned in close accord with man’s cognitive activity.(Catholic Humanism and Modern Letters, 1954)

the central fact [of the identity] of human cognition and the artistic process (…) [is] the key to the modern world. (Catholic Humanism and Modern Letters)

*

the Symbolists [took] aesthetic experience as an arrested moment (…) for which (…) they sought the art formula by retracing the stages of apprehension which led to this moment. (Tennyson and Picturesque Poetry, 1951)

Compare from ‘Catholic Humanism and Modern Letters’ (1954):

The poetic process is a reversal, a retracing of the stages of [ordinary]³ human cognition. It has and will always be so; but with Edgar Poe and the symbolists this central human fact was taken up to the level of conscious awareness. It then became the basis of modern science and technology. That is what Whitehead meant when he said that the great event of the nineteenth century was the discovery of the technique of discovery.

 Hence (in the same place): 

The rational notes (…) traced by St. Thomas were actual stages of apprehension in every moment of human awareness.

It is imperative to pay close attention to the action of genitives in McLuhan’s work, especially in regard to their relations with time. “The stages of apprehension” in the above passage from ‘Tennyson and Picturesque Poetry’ is a subjective genitive, not an objective one. That is, the “stages” belong to “apprehension” as their possessing subject (like ‘the ball of the boy’); they are not the reverse where “apprehension” would be the genitive object of the “stages” (like ‘the manufacture of the car’).

“Apprehension” is inclusive of its temporal stages and has actively organized them, it is not organized by them passively in an external or exclusive manner.

The claim is that “apprehension” is not fabricated through some assembly line process (as if it were the object resulting from compositional “stages”), although this linear notion has been assumed by most philosophy and psychology since Descartes. The supposition has been that experience ‘begins’ with some or other sensory input (external or internal) and then is individually and culturally shaped in a kind of customizing process through the application of categories or filters. This is so with Kant as much as with Freud. Instead, says McLuhan, while experience is indeed generated through a temporal process, the time of its genesis is not “sequential” or diachronic, but “simultaneous” or synchronic:

Time considered as sequential (…) is figure and time considered as simultaneous (…) is ground. (The Global Village)⁴

Hence, as cited above from ‘Catholic Humanism and Modern Letters’, he could refer to:

stages of apprehension in every moment of human awareness.

The whole “apprehension” with its “stages” is already composed, and is already available, but must be selected (so to say) from the panoply of other whole apprehensions which are equally already composed and already available. To compare: when we speak, our words are not fabricated in “stages” of one sound or one syllable added to another in a chronological process, but are available as already composed in their complex “stages” — along with all sorts of alternate words and expressions with their complex “stages”. Below and before the chronological sequence of our spoken words, there is a synchronic constitution of grammar and a selective operation on it that recognizably expresses both individual personality and social membership (age-group, education, class, region, nationality, etc), in addition (it may be) to some or other semantic intention. 

For McLuhan it was just this language process in simultaneous depth which is the elementary form, or “essential type”, constituting the unperceived environment to all human action.⁵

In regard to our own experience and behavior, individual and social, we remain in the same situation as was the world between whenever it was that human being⁶ originated and 1800 (say), when it began to dawn on us that we live in a physical environment constituted by chemical elements. This was an environment that had always and everywhere been active, and that always and everywhere will remain active, but had never before been perceived. It was a total environment of the farthest reaches of the universe, and of the nearest reaches of our own bodies, that had never before been known to exist. The revolutionary changes to the planet in the last 200 years have resulted from this new consciousness of our perennial physical environment, the ‘exterior landscape’.

It was McLuhan’s hope and prediction that an analogous new consciousness of our perennial ‘interior landscape’ could lead to changes of a similar scope.⁷ And it was here alone, he thought, that the way to peace might be found for a world currently shaped by an unknown and out of control interior environment⁸ (perilously combined with an exterior environment that had become capable of nuclear war). 

the esthetic discoveries of the symbolists since Rimbaud and Mallarmé (developed in English by Joyce, Eliot, Pound, Lewis and Yeats) (…) have served to recreate in contemporary consciousness an awareness of the potencies of language…⁹ (McLuhan to Harold Innis, 1951)

[The imperative need today is] to understand the techniques and functions of the traditional arts as the essential type of all human communication. (ibid)

One major discovery of the symbolists which had the greatest importance for subsequent investigation was their notion of the learning process as a labyrinth of the senses and faculties whose retracing provided the key to all arts and sciences (…) Retracing becomes (…) the technique of reconstruction.¹⁰ (…) From the point of view of the artist (…) the business of art is no longer the communication of thoughts or feelings which are to be conceptually ordered, but [to facilitate] a direct participation in an experience. The whole tendency of modern communication whether in the press, in advertising, or in the high arts is toward participation in a process, rather than apprehension of concepts. And this major revolution, intimately linked to technology, is one whose consequences have not begun to be studied although they have begun to be felt. (ibid)

It is popular (…) to attack advertising. But is is unheard of to take it seriously as a form of art. Personally I see it as a form of art. And like symbolist art it is created to produce an effect rather than to argue or discuss the merits of a product. Baron Wrangel, the man in the Hathaway shirt [advertisement] — white shirt and black eye-patch: what did it mean? Out of the millions who bought Hathaway shirts, how many could say what the ad meant? It was a piece of magic: irrational, meaningless. But it had a definite effect. The advertiser proclaims to his clients that his pictorial and verbal magic is linked to the assembly line. No pictorial magic, no mass production. The primitive witch-doctor had spells which controlled the elements. The modern advertiser concocts spells which compel the customer. What the advertisers have discovered is simply that the new media of communication are themselves magical art forms. All art is in a sense magical in that it produces a change or metamorphosis in the spectator. It refashions his experience. In our slap-happy way we have released a great deal of this magic on ourselves today. We have been changing ourselves about at a great rate like Alley Oop. Some of us have been left hanging by our ears from the chandeliers. (Catholic Humanism and Modern Letters, 1954)