McLuhan frequently identifies “existence” or “being itself” as his topic:

To the alerted eye, the front page of a newspaper is a superficial chaos which can lead the mind to attend to cosmic harmonies of a very high order. (The Mechanical Bride, 4)

We are suddenly eager to have things and people declare their beings totally. There is a deep faith to be found in this new attitude — a faith that concerns the ultimate harmony of all being. Such is the faith in which this book has been written. UM (1964) 

I have a deep and abiding belief in man’s potential to grow and learn, to plumb the depths of his own being and to learn the secret songs that orchestrate the universe. We live in a transitional era of profound pain and tragic identity quest, but the agony of our age is the labor pain of rebirth. Playboy Interview (1969)

Here is a chronological sampling of the “sense of Being” (dual genitive) in his work:

Whereas the ethical world of Ulysses is presented in terms of well-defined human types the more metaphysical world of the Wake speaks and moves before us with the gestures of being itself. It is a nightworld and, literally, as Joyce reiterates, is “abcedminded”. Letters (“every letter is a godsend”), the frozen, formalized gestures of remote ages of collective experience, move before us in solemn morrice¹. They are the representatives of age-old adequation of mind and things, enacting the drama of the endless adjustment of the interior acts and dispositions of the mind to the outer world. The drama of cognition itself. For it is in the drama of cognition, the stages of apprehension, that Joyce found the archetype of poetic imitation. He seems to have been the first to see that the dance of being, the nature imitated by the arts, has its primary analogue in the activity of the exterior and interior senses. James Joyce: Trivial and Quadrivial (1953)

In ordinary perception men perform the miracle of recreating within themselves — in their interior faculties — the exterior world. This miracle is the work of the Nous Poietikos or of the agent intellect — that is, the poetic or creative process. The exterior world in every instant of perception is interiorized and recreated in a new matter. Ourselves. And in this creative work that is perception and cognition, we experience immediately that dance of Being within our faculties which provides the incessant intuition of Being. I can only regard the movie as the mechanization and distortion of this cognitive miracle by which we recreate within ourselves the exterior world. But whereas cognition provides that dance of the intellect which is the analogical sense of Being, the mechanical medium has tended to provide merely a dream world which is a substitute for reality rather than a means of proving reality. Catholic Humanism and Modern Letters (1954)

The interval is the means of epiphany or revelation. It is the release which Hopkins called Sprung Rhythm. It is the instrument of analogical intuition of Being. Verbi-Voco-Visual Explorations (1957)

it was not till the pre-Raphaelites and Hopkins that a deliberate campaign for Saxon tactile values in language was to begin in English. Yet tactility is the mode of interplay and of being rather than of separation and of lineal sequence. GG (1962)

For with the isolation of the visual, the feeling of interplay and of light through the mesh of being yields, and “Human thought no longer feels itself a part of things.” GG (1962)

symbolism strove to recover the unified field of being once more… GG (1962)

Just before an airplane breaks the sound barrier, sound waves become visible on the wings of the plane. The sudden visibility of sound just as sound ends is an apt instance of that great pattern of being that reveals new and opposite forms just as the earlier forms reach their peak performance. UM (1964)

we’re standing on the threshold of a liberating and exhilarating world in which the human tribe can become truly one family and man’s consciousness can be freed from the shackles of mechanical culture and enabled to roam the cosmos. I have a deep and abiding belief in man’s potential to grow and learn, to plumb the depths of his own being and to learn the secret songs that orchestrate the universe. We live in a transitional era of profound pain and tragic identity quest, but the agony of our age is the labor pain of rebirth. Playboy Interview (1969)

the word medium in Aquinas refers to the gap or interval, the emptiness between matter and form as such, i.e., the hidden ground of Being, and in every sense, it is the message. MM to Fr Gerald Pocock (May 7, 1976)

There is [for an oral tradition] no past or future, only the essence of being that exists now. GV (posthumous)

the uttered logos (…) embedded in things animate and inanimate which structures and informs them and provides the formal principles of their being…  GV (posthumous)