Mallarmé (…) saw that a poetry of effects was impersonal. The author effaced himself above all in not assigning causes or explanations as transitional devices of a novelistic and a pseudo-rationalistic type between the parts of a poem. (Mr. Eliot’s Historical Decorum, 1949) 

When a new environment forms, we see the old one as if we lived in a world of the déjà vu. (…) Yet this strategy merely ensures that whenever we encounter the unfamiliar, we will translate it into something we already know. It is this that seems to make the present almost impossible to apprehend in any period or culture. It was James Joyce in Finnegans Wake who demonstrated that the way to overcome the fear of the present, and of innovation in general, is to make an inventory of all the effects of the new thing as it encounters all the older forms of the society. (The Future of Morality: inner vs outer quest, 1967)

It was the symbolist poets who began the study of effects minus causes. This is a technique indispensable to the developing of perception and the by-passing of concepts. (McLuhan to Jim Davey, March 22, 1971)

the discovery of the “technique of discovery” (…) is [to learn how] to trace a process backward from its ultimate effect to the point at which to begin to produce that effect, i.e., to invent the market before the product. This was the discovery of Poe in detective fiction, and Baudelaire in poetry. (Take Today, 195)

This putting the effect before the cause is what we do typically and ordinarily in the electric time.  In 1844, at about the same time that [Georg Cantor, 1845-1918],¹ the mathematician, invented set theory by separating the mathematical operations from mathematical quantities, Edgar Allan Poe, the great innovator in the arts, separated the poetic process from poetry. This was his great breakthrough, and it was of instant effect on the French symbolists and the French poetic activity of the period. Baudelaire translated Poe (…) and took on this idea of simultaneity that if you want to write a poem you have to start with the effect and then look around for the causes. And this became the awareness of acoustic space in which the beginning and the end are at the same time. This is the kind of space and time in which we live now. Einstein was only catching up with Poe in the twentieth century when he invented space-time or relativity theory. The poets and the artists are usually fifty years ahead of the physical scientists in devising models of perception. The job of the artist is to devise means of perceiving that are relevant to the situation in which you exist. This is the gap between biology and technology… (Art as Survival in the Electric Age, 1973)

Poe hit upon the key to the electric age, programming from effects in order to anticipate causes. (Art as Survival in the Electric Age, 1973)

I begin with effects and work round to the causes, whereas the conventional pattern is to start with a somewhat arbitrary selection of ‘causes’ and then try to match these with some of the effects. It is this haphazard matching process that leads to fragmentary
superficiality. (McLuhan to Franklin R Gannon, June 12, 1973, Letters 478)

converts come in through the back door of the church. Coming in through the back door is coming in through the effects of the church, and not through its teachings. (McLuhan to Nina Sutton, 1975)

In the late 1940s a series of influences suddenly coalesced for McLuhan into a position he would continue to articulate for the remaining thirty years of his life.  This coalescence amounted to his second conversion. The central notion was that everything experienced in human life and culture is contingent effect, never cause — but that contingent effect indirectly suggests cause (via induction and making, not deduction and matching).

In the order of their work on (working over) McLuhan, these influences were: T.S. Eliot’s lectures and essays and especially his Four Quartets which appeared sequentially in the late 1930s and early 1940s; the many books and essays of Etienne Gilson from the 1920s and 1930s²; the work of Sigfried Giedion and Wyndham Lewis, both of whom McLuhan met in 1943; Edgar Poe’s ‘Descent into the Maelstrom’ (1841) and ‘Philosophy of Composition’ (1846); Cleanth Brooks’ poem on the Maelstrom explicitly correlating it with times plural (written in 1944 and published in 1946)³; the poems and essays of Stéphane  Mallarmé (1842-1898); the essays and poetry of Ezra Pound; and the novels of James Joyce (which McLuhan reread in the late 1940s).⁴

When in 1938 Bernard Muller-Thym returned with his PhD from the University of Toronto to St Louis University to teach in the philosophy department, he and McLuhan rapidly became close friends. Muller-Thym would be the best man at McLuhan’s marriage in 1939 and the Godfather of McLuhan’s first two children, Eric (b 1942) and Mary (b 1944 with twin Teri). In Toronto Muller-Thym had been Etienne Gilson’s favorite student and in turn passed on his knowledge of Gilson’s work to McLuhan. The years Muller-Thym and McLuhan spent together in St Louis (before Muller-Thym enlisted in the navy in 1942) amounted in this way to a master class in Gilson’s thought conducted by Muller-Thym for McLuhan. As is especially evident once the editor’s additions to McLuhan’s own bibliography are discounted, Gilson would became by far the most cited reference in McLuhan’s PhD thesis on Nashe, which was submitted and approved in 1943. 

Gilson’s teaching on the chronological⁵ and phenomenological precedence of effect to cause is especially treated in his 1930 essay, ‘The Future Of Augustinian Metaphysics’:⁶ 

The fact on which [Augustine] fastened as the witness in ourselves of the existence of God was the true judgement. His analysis, often repeated, of the characteristics (…) which formally define truth as such, is well known; the antinomy between the contingency of the subject as the vehicle of truth on the one side, and the necessity of truth itself, whatever  its object, on the other, can only be solved by the admission of a subsistent truth [or medium] (297)

For the Augustinian proof to have its full effect, it is necessary that, somehow or other, the human intellect, which conceives the truth [in the sciences, but also in everyday understanding of language and the environment], should not be the immediate sufficient cause of its truth;⁷ if it is, there is no necessity for it to affirm the existence of God as cause, and then the way opening [to God] through thought is blocked at the very entrance. Doubtless there would remain the search for God in the order of causality, as cause of the intellect itself (which Albert the Great was to attempt), but St. Augustine does not even try this, because the only operation of the intellect which requires the affirmation of God as its sufficient reason is the [existence]⁸ of truth. He has, therefore, always to come back to the true judgement, or, what comes to the same thing, the intellect, so far as it is capable of conceiving truth. (298)

if (…) this is the point on which his proof rests, it must necessarily follow that Divine illumination (to give it its traditional name) must reach thought directly [as productive cause, but known as direct cause only indirectly through effect]. For either it reaches it directly and in that case we grasp at the same time the sufficient reason of truth and God who is its foundation; or it reaches it indirectly, and in that case we are equally incapable of attaining to the existence of God and of accounting for truth [since in this case these would depend from the recognition bestowed upon them by our contingency]. (298-299)

To say that “we know in God”, or that we see His hidden light,⁹ is this not tantamount to inviting the metaphysician in search of a mystical intuitionism to treat God as the very light of our thought, as the natural and first object of this thought, so that, instead of knowing Him through things, we know things through Him? This deviation begins from the end of the twelfth century onwards, under the influence of Arabic neo-Platonism, and especially of Avicenna; [although] dammed in by the efforts of St, Bonaventura and of St. Thomas Aquinas, it spreads in the seventeenth century with Malebranche, thanks to the influence
of Cartesian idealism, and reaches its height in the nineteenth century under the impulse of German idealism. (299)

In proportion as the teaching of St. Augustine aimed at being a metaphysic, it is a metaphysic based upon a psychological empiricism, or, if preferred, a metaphysic of inner experience. Hence its extreme suppleness, its power of rebirth, and the very incompleteness which left a permanent possibility of progress open before it. (302-303)

The important point before all else is to understand that the two philosophies [Augustinian and Cartesian] have no essential relation (…) What is, for the French philosopher, but the initial step in a regulated order of thoughts is for St. Augustine a concrete and painful experience, an illness from which he has suffered and of which he has cured himself.(…) si fallor sum (301, 303)

Assuming (…) St. Augustine’s method is as we have described it, what do we find as the necessary starting-point of our search? Facts, and nothing but facts. These facts may be, and often are, facts of inner experience, they may be ideas — but ideas taken not as principles of deduction, but as the basis of induction. The problem of the existence of God enjoys no privileged position in his teaching. It is, indeed, a unique case in respect of the reality at stake and consequently  also in respect of the nature of the datum which allows us to attain to it, but this datum differs from other data only in content, not in nature. Like being, like life, like sensation, like thought, truth is a fact; like other facts, it is presented to our empirical observation; like other empirical  observations, it demands of metaphysics the discovery of its sufficient reason; and if God alone can furnish its sufficient reason, we shall have proved the existence of God. Nothing here ever leaves the strictly philosophical order to pass over into mystical intuition and to substitute it for philosophical thought. (305-306)

Every ontological interpretation of St. Augustine presupposes, then, a more or less complete misunderstanding of his radical empiricism. (…) The primum cognitum of St. Augustine is not God; it is man within the universe, and, within this universe and this man, the experience of a true judgement. But it must be added that this primum cognitum is not (…) the primum reale; on the contrary, it [the primum cognitum] becomes intelligible only on condition of finding its sufficient reason in a transcendent fact which provides its explanation.  (306)

St. Augustine starts from a complex cognitum in which he distinguishes by analysis an order of reality which postulates in its turn that of the First Being. Once this Being is apprehended and posited, it becomes possible to set off into an order which is not that of deduction, but rather of production; and even then it must be remembered that the start is taken not from a principle, but from the consequence, since we ourselves are only a consequence (…) the doctrine of divine illumination is not the vision of the First Cause, but the induction of the First Cause, starting from an effect, namely [the fact of our knowledge of] truth. (306-307)

The congenital impotence of our intellectual light to apprehend truth, a correlative impotence of our will to compass the good until truth and goodness are accepted as the gifts of God, instead of being conquered like the spoils of the victor, had been St. Augustine’s experience…¹⁰ (307)

to be Christian qua philosophy, a philosophy must be Augustinian or nothing. His metaphysic of nature completes a metaphysic of grace, because nature is given to the Christian in grace, which, working in him inwardly, manifests itself there in the manner of a cause
revealed by its effects. (308)

It is in no case possible for man to start from God to deduce from Him the creature; on the contrary, he must mount from the creature to God. The course recommended by St. Augustine — and herein lies his personal contribution to the treasure of tradition — is the path to God, leading through this particular creature which is man, and in man, thought, and in thought, truth. But this means, quite beyond  speculations about the nature of truth and its metaphysical  conditions, a sort of moral dialectic that, taking as object of its search the search itself by man of God, endeavours to show the presence in the heart of man of a contingency… (312)

that secret door behind which God stands. (313)

a renewed Augustinianism (…) would have to become assimilative and creative (…) it will so become, once it realizes that its function is to do well what has been badly done by modern idealism, to re-establish it on the foundations of a psychological realism which is its natural basis…  (314)

In a word¹¹, “that secret door behind which God stands” is the utter finitude and contingency of everything human — a finitude and contingency that is yet somehow capable of communication and of learning truth.  These capabilities taken as effects or contingencies are, once intensely interrogated, revelatory of what is before them as cause — and so makes them possible in a kind of knotted feedback-feedforward action.

This is, of course, circular — from what is later, what is earlier is induced.  But being subject to this circularity is just what human being is.