the emotion of multitude (…) is a state in which we live constantly, that is, on the border. We live constantly in two worlds… (Canada: the Borderline Case, 1967)

the principle of poetic organization is not narrative but interface based on the resonant interval. Yeats refers to it as the “emotion of multitude”, and Joyce calls (…) language the mirror of the mind of man, the square wheel without spokes which encompasses all cycles of human experience in a simultaneous present. (McLuhan, Discontinuity and Communication in Literature, 1970)

In 1903, W. B. Yeats, meditating on the “emotion of multitude,” explained that it is achieved by a discontinuous parallel between two actions (…) Depth awareness is created by parallel suggestion, not by connected statement. (McLuhan to The Listener, 1971)¹

How did I arrive at these “Laws of the Media”? By a structural approach. The structuralists, beginning with Ferdinand de Saussure and now Lévi-Strauss, divide the approaches to the problem of form into two categories: diachrony and synchrony. Diachrony is simply the developmental, chronological study of any cultural matter; but synchrony works on the assumption that all aspects of any form are simultaneously present in any part of it. Although I have used the simultaneous approach in arriving at these Laws of the Media, any one of them is susceptible to the diachronic approach for filling in the historical background and details. Since electric speeds of information constitute a sort of simultaneous structuring of experience, synchrony, representing all directions at once, is, as it were, acoustic; whereas the diachronic, representing one stage at a time, is visual in its analytical pattern. (McLuhan’s Laws of the Media, 1975)² 

Here is Claude Lévi-Strauss in 1949:³

anthropology cannot remain indifferent to historical processes and to the most highly conscious expressions of social phenomena. But if the anthropologist brings to them the same scrupulous attention as the historian, it is in order to eliminate, by a kind of backward course, all that they owe to the historical process and to conscious thought. His goal is to grasp, beyond the conscious and always shifting images which men hold, the complete  range of unconscious possibilities. These are not unlimited, and the relationships of compatibility or incompatibility which each maintains with all the others provide a logical framework for historical  developments, which, while perhaps unpredictable, are never arbitrary. In this sense, the famous statement by Marx, “Men make their own history, but they do not know that they are making it,” justifies, first, history and, second, anthropology. At the same time, it shows that the two approaches are inseparable. (…)
It would be inaccurate, therefore, to say that on the road toward the understanding of man, which goes from the study of conscious content to that of unconscious forms, the historian and the anthropologist travel in opposite directions. On the contrary, they both go the same way. The fact that their journey together appears to each of them in a different light — to the historian, transition from the explicit to the implicit; to the anthropologist, transition from the particular to the universal — does not in the least alter the identical character of their fundamental approach. They have undertaken the same journey on the same road in the same direction; only their orientation is different. (…) A true two-faced Janus, it is the solidarity of the two disciplines that makes it possible to keep the whole road in sight.

Lévi-Strauss put his finger here on a series of fundamental points which were basic also to McLuhan (despite the seeming wide disparity of their work) and at exactly this same time around 1950 (when McLuhan was fascinated by the multi-leveled epyllion⁴ form and was introduced to Innis’ structuralism):

• There are two realms each with their own time (one particular-historical-conscious-actual and the other universal-logical-unconscious-possible) which must be understood both in their difference and in their interconnection — the latter realm supplying the “framework” for the former and the former realm being the always particular expression of the latter.  A science of human being — Lévi-Strauss’s “anthropology” — cannot have a different structure from the physical sciences, all of which have this same “two-faced Janus” relationship between levels of theory and factual instance which are yet knotted in their mutual “solidarity”.
• Because there cannot be an understanding of a ‘part’ aside from the ‘whole’ of which it is a part, both history and its “framework” must be grasped in their “complete range”. To compare, chemistry approaches all of material being with a complete theory of its elements and their interactions. But both this chemical theory and the material being which is the object of its investigations remain radically open. Indeed, it is a central aspect of the birth of any science to revise for its domain just what ‘complete’ and ‘incomplete’ entail in it and how they relate to each other there. A ‘complete’ explanation is suddenly seen — but the investigation of that ‘complete’ explanation, if it turns out to be valid, can and will go on forever since it is always also incomplete.
• Because reality has this dynamic possible/actual structure, so must investigation of it be correspondingly “structural” (relational, double, a ratio). And this is especially the case in the sciences of human being where the study itself always amounts to some representation of an individual or collective representation.   

Now McLuhan was aware⁵ of these points very early — in his late twenties and early thirties around 1940 when he was teaching at St Louis University. In his first publication there, in 1938, describing ‘The Cambridge English School’ he was explicit about the plurality and multiple levels of time:

Donne, and the later Shakespeare, on the one hand, and the Romantics on the other, have been read at Cambridge as though they were contemporaries of Mr. Eliot — which of course they are. For the continuing life of the language itself is such as to constitute a medium in which they are all contemporary.

Here “language itself” as a “medium” which is always “contemporary” is just Lévi-Strauss’s “complete range of unconscious possibilities” supplying “a logical framework for historical  developments”. 

A couple years later (probably in 1941, although publication followed only in 1944) he ended his paper on ‘Poetic and Rhetorical Exegesis: The Case for Leavis against Richards and Empson’⁶ by characterizing Leavis’ work as implementing:

the program which Mr. Eliot (…) indicated but relinquished (…) the arduous stage of the journey which remains to be accomplished before winning an overall view, which is plenary critical judgment.

Here again, then, is to be seen the need for “overall” or “plenary” specification of the “complete range of unconscious possibilities” which would supply “a logical framework for historical  developments” — and hence the springboard for the sort of “critical judgment” made, constantly and universally, in every science.⁷

McLuhan did not know it at the time, but he would come to see W.B. Yeats as the modern progenitor of this insight in his 1903 ‘Emotion of Multitude’.⁸ In a 1970 lecture, ‘Discontinuity and Communication in Literature’,⁹ McLuhan read the complete 2-page text of ‘Emotion of Multitude’ by way of saying, ‘Here is the font — consider it well’: 

I [WBY] have been thinking a good deal about plays lately, and I have been wondering why I dislike the clear and logical construction which seems necessary it one is to succeed on the modern stage. It came into my head the other day that this construction, which all the world has learnt from France, has everything of high literature except the emotion of multitude. The Greek drama has got the emotion of the multitude from its chorus, which called up famous sorrows, even all the gods and all heroes, to witness as it were, some well-ordered fable, some action separated but for this from all but itself. The French play delights in the well-ordered fable, but by leaving out the chorus, it has created an art where poetry and imagination, always the children of far-off multitudinous things, must of necessity grow less important than the mere will. This is why, I said to myself, French dramatic poetry is so often a little rhetorical, for rhetoric is the will trying to do the work of the imagination. The Shakespearian drama gets the emotion of multitude out of the sub-plot which copies the main plot, much as a shadow upon the wall copies one’s body in the firelight. We think of KING LEAR less as the history of one man and his sorrows than as the history of a whole evil time. Lear’s shadow is in Gloucester, who also has ungrateful children and the mind goes on imagining other shadows, shadow beyond shadow, till it has pictured the world. In Hamlet, one hardly notices, so subtly is the web woven, that the murder of Hamlet’s father and the sorrow of Hamlet are shadowed in the lives of Fortinbras and Ophelia and Laertes, whose fathers, too, have been killed. It is so in all the plays, or in all but all, and very commonly the subplot is the main plot working itself out in more ordinary men and women and so doubly calling up before us the image of multitude. Ibsen and Maeterlinck have on the other hand created a new form, for they get multitude from the Wild Duck in the Attic, or from the Crown at the bottom of the Fountain, vague symbols that set the mind wandering from idea to idea, emotion to emotion. Indeed all the great masters have understood that there cannot be great art without the little limited life of the fable, which is always the better the simpler it is, and the rich far-wandering many imaged life of the half-seen world beyond it. There are some who understand that the simple unmysterious things living as in a clear noonlight are of the nature of the sun, and that vague, many-imaged things have in them the strength of the moon. Did not the Egyptian carve it on emerald that all living things have the sun for father and the moon for mother, and has it not been said that a man of genius takes the most after his mother?

How early McLuhan had first set off on this way may be seen in texts we have from 1934:

There are (…) in all times and places, definite types of temperament displaying consistency of conformation. (McLuhan’s University of Manitoba M.A. thesis on Meredith)¹⁰

Of late I have been wayfaring among the work of T.S. Eliot (…) the poems I am reading have the unmistakable character of greatness. They transform, and diffuse and recoalesce the commonest every day occurrences of 20th century city life till one begins to see double indeed — the extremely unthinkable character, the glory and the horror of the reality in life… (McLuhan letter to his family from Cambridge, Dec 5, 1934)¹¹

The need, specified by here by McLuhan when he was 23, is just that of Lévi-Strauss: “to grasp, beyond the conscious and always shifting images which men hold, the complete range of unconscious possibilities”, extending from “glory” to “horror”. But this not in some spectrum separated from life, but is exactly “the reality in life”, as perceived through a near “unthinkable” focus that is “double indeed” and yet at the same time “recoalesce[d]”. As Lévi-Strauss has it in ‘History and Anthropology’ (full passage above):

A true two-faced Janus, it is the solidarity of the two disciplines that makes it possible to keep the whole road in sight.