The reason that Joyce considered Vico’s new science so important for his own linguistic probes, was that Vico was the first to point out that a total history of human culture and sensibility is embedded in the changing structural forms of language. (McLuhan to Robert Leuver, Jul 30, 1969, Letters 384; also, Medium and the Light 89)

McLuhan took it, along with a long tradition, that language is what distinguishes human beings from other beings. It followed that if we want to know what human being is, we must learn what language is. But if language and human experience were interrelated from the start (“the first stage of apprehension is already poetic” as McLuhan wrote Pound already in 1951, Letters 229), the analysis of experience as an analogous mode of investigation to linguistic analysis lay close at hand.

Now the exercise of language may be analyzed as presupposing choices among available ‘contesting’ sounds and grammatical markers.  But these ‘choices’ are, of course, neither conscious, in the main, nor situated in normal time and space.  When we speak, these choices have always already been made — but when and where and by whom they have been made remains utterly obscure.

McLuhan studied human experience in a comparable way:

it is impossible that there could ever be a scientific concept that is not embedded in the vernacular tongue of the scientist, and that has not been embedded there for many centuries. You cannot conceive a form of scientific hypothesis which is not part of your own language, implicit in that language. All the mathematics in the world are externalizations of certain linguistic patterns. What the poets were saying — now more widely appreciated — was that the language itself embodies the greatest body of scientific intuition possible. The proportionalities in things, and between things and our senses, and so embodied in language itself, are inexhaustible. The particular technology of a time releases some of that inexhaustible store of analogical intuition and experience which IS language. So television releases within language a whole body of resources which has been bound up there for centuries. But this does not depend upon concepts. It has to do with sensibility and observation — analogical perception, right in the structure of language itself. ‘Communications and the Word of God’, 1959)¹

All of man’s artifacts are structurally linguistic and metaphoric. This discovery, unknown to anybody in any culture, would justify a book without any other factors whatever. Remember the [James] Watson autobiography of his discovery of the double helix in the DNA particle? Literally speaking, this breakthrough about the linguistic structure of all human artifacts is incomparably larger and deeper-going. I am, myself, unable to grasp the implications. Certainly it means that the unity of the family of man can be seen, not as biological, but as intellectual and spiritual. (McLuhan to Barbara Rowes, April 29, 1976)²

The model of explanation at work here is that of experiential phenomena as figures overlying their ground (just as chemistry, say, envisions physical materials as figures overlying the chemical elements as their ground). This is already a two-fold structure. But just as chemical elements, in turn, have further structure of their own (the ratio of protons and electrons, say), so McLuhan imagined that the grounding elements of language and experience are themselves structured:

Structuralism as a term (…) [designates] inclusive synesthesia, an interplay of many levels and facets in a two-dimensional mosaic. (GG, 230)

This, he said, was the

principle of a continuous dual structure for achieving order. (Spiral — Man as the Medium, 1976, 126)

“Continuous” in this context is multi-dimensional:

• there are no experiential phenomena which lack ground — that is, everything experienced is a figure for which grounding is always in place, ‘continually’…
• the structure of ground is ‘continuous’ in another sense, as, for example. a sample of pure gold, although having a single elementary structure, is not one atom but a great many atoms, each with the same ‘continuous’ structure…
• if such material is to cohere, as a pure gold nugget (say), or as a compound lump or mixture of many elements, there must be a further structure ‘continually’ holding these repeated atomic structures together…

The explanatory structure of experience McLuhan had in mind was therefore dynamic in many ways: between figure and ground, between the poles (eg, eye and ear or space and time) of the experiential elements, and between the experiential elements themselves. Any of these might be flipped at any time.  The key was exposed, as McLuhan wrote in the very first words of his very first paper in January 1936, “when it is seen that there are two principal sides to everything…” (‘G. K. Chesterton: A Practical Mystic’).