Giedion influenced me profoundly. Space, Time and Architecture (1941) was one of the great events of my lifetime. (Stearn interview)

In Space, Time and Architecture (1941), Giedion makes a series of points which McLuhan would find decisive for his life’s work:

Giedion: “in spite of seeming confusion, there is nevertheless a true, if hidden, unity, a secret synthesis, in our present civilization.” (Space, Time and Architecture, Foreword to the first edition.)

McLuhan: “There is a real, living unity in our time, as in any other, but it lies submerged under a superficial hubbub of sensation. (McLuhan to Harold Innis, March 14 1951, Letters 223)¹

Giedion: “To point out why this synthesis has not become a conscious and active reality has been one of my chief aims.” (Space, Time and Architecture, Foreword to the first edition.)

McLuhan:  “it has been the effort of this book to explain how the illusion of segregation of knowledge had become possible by the isolation of the visual sense by means of alphabet and typography.” (The Gutenberg Galaxy)

In the latter passages, both Giedion and McLuhan describe the “aim” or “effort” of their most important books. Giedion investigates a missing “synthesis”; McLuhan investigates the flip side of the same coin, “the illusion of segregation”, “the isolation”.

Giedion: History is not a compilation of facts, but an insight into a moving process of life. (Space, Time and Architecture) 

McLuhan: “History is not a compilation of facts, but an insight into a moving process of life.” S. Giedion, Space, Time and Architecture (Take Today, 76)

There is an original complexity (“a moving process of life“) in which all human events unfold. This complexity accounts for both finitude (as an original going out) and finitude’s relation beyond itself (as an original going in). In McLuhan’s terms: “Un Coup de Des [“Throw of the Dice”] illustrates the road he [Mallarmé] took in the exploitation of all things as gestures of the mind, magically adjusted to the secret powers of being.”  (Joyce, Mallarmé and the Press, 1954)

Giedion: In Civilization of the Renaissance Burckhardt emphasized sources and records rather than his own opinions. He treated only fragments of the life of the period but treated them so skillfully that a picture of the whole forms in his readers’ minds. (…) Modern artists have shown that mere fragments lifted from the life of a period  can reveal its habits and feelings; that one must have the courage to take small things and raise them to large dimensions. (STA, 3-4)

McLuhan: Now was the time for the artist to intervene in a new way and to manipulate the new media of communication by a precise and delicate adjustment of the relations of words, things, and events. His task had become not self-expression but the release of the life in things. (…) As a vacuum tube is used to shape and control vast reservoirs of electric power, the artist can manipulate the low current of casual words, rhythms, and resonances to evoke the primal harmonies of existence… (Joyce, Mallarmé and the Press)

Two further STA texts show Giedion’s general influence on McLuhan which would only be diminished by giving specific McLuhan texts in proof:

Giedion: These artists have shown in their pictures that the furniture of daily life, the unnoticed articles that result from mass production — spoons, bottles,  glasses, all the things we look at hourly without seeing — have become parts of our natures. They have welded themselves into our lives without our knowing it. (4)

Giedion: Moreover, such insight is obtained not by the exclusive use of the panoramic survey, the bird’s-eye view, but by isolating and examining certain specific events intensively, penetrating and exploring them in the manner of the close-up. This procedure makes it possible to evaluate a culture from within as well as from without. (vi)²

While individual quotations might certainly be found to parallel these STA passages, in fact McLuhan’s whole work might be considered as arising from them (although not only from them, of course) and as framed by them.