And just as our individual experiences of our individual senses get processed by some sort of inner common sense which gives unity to the diversity of our sensations, so with the media as extensions of our senses. These cooperative technological extensions of ourselves undergo a social or communal processing which gives them unity… (Explorations 8, 1957, 1-18)

A sensus communis for external senses is what I’m trying to build. (McLuhan to Walter Ong on November 18, 1961, Letters 281)

Between these two dates (in the first of which ‘unity’ is a social fact, but in the second remains outstanding), presumably in the course of composing his Project in Understanding New Media, McLuhan seems to have realized that it was not enough to attest to unity as he and Giedion did:

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, 1941.)

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)

Instead, he now understood (at a time when his mother’s health was fatally broken¹ and his own was starting to break with “occasional blackouts and dizziness” (Letters, 175) and his children were falling away from the church) that the perception of unity was problematic (whatever its metaphysical status might be).  Further, that this failure of perception had potentially devastating effect when human destructive power was greater than ever and the world was smaller than ever. 

It may be guessed that his wrestling with “acoustic space” — which he would continue from 1955 until his death 25 years later — was above all motivated by the question of how to articulate a discontinuous and non-linear ‘unity’ in order to promote its renovated perception. And this at a time when, beyond the perennial problems posed to such perception by human mortality and evil and blindness, in the new situation of the electric era all traditional perspectives, with their associated particular conceptions of unity, were doomed.