Electronic media (…) abridge space and time and single-plane relationships, returning us to the confrontation of multiple[-plane] relationships at the same moment. (…) The multilayered montage or “transparency,” with its abridgement of [sequential] logical relationships, is as familiar in the cave painting as in cubism. (Myth and Mass Media, 1959)¹ 

this fusion and telescoping of phases of process becomes a kind of explanation or mode of intelligibility. (Myth and Mass Media)

McLuhan had always been interested in the subliminal working of advertising and of other sorts of education outside the classroom like comics and the movies. This was the “classroom without walls”. But in the later 1950’s he became interested in the subliminal in a new way.

For 500 years our idea of efficacy and efficiency was rooted in the technology of explicitness. To make happen and to explain scientifically have both meant the consecutive spelling out of consequences, one at a time. In the electronic age we enter the phase of the technology of implicitness in which by grasp of total field relationships we package information and deliver messages on many levels, all in an instant. (‘The Subliminal Projection Project’, The Canadian Forum, December, 1957)

He now realized that the subliminal was not only something we needed to become aware of through “the consecutive spelling out of consequences”. This latter sort of investigation could certainly expose subliminal messages suggested by ads (for example) and could therefore contribute to consumer insight. But such ‘subliminal’ messages were potentially fully conscious and this was, indeed, just what study of them aimed to bring about: the conversion or “spelling out” of the subliminal into the “consequence” of conscious awareness.

But the subliminal could also to be investigated as being part of “total field relationships (…) on many levels, all in an instant.” And this was an entirely different matter.

Compare brain chemistry.² The subliminal working of molecules in the brain is not something to be brought into consciousness in the same way as the exposure of hidden messages in ads. Instead, that molecular working can certainly be investigated in labs, and even be imaged through microscopy or MRI, but the object of this sort of highly conscious endeavor is not at all to bring that brain activity into the consciousness with which we go about our daily business. Indeed, the former can never become part of the latter because its working takes place, instantaneously, on a different level. As McLuhan put it, we have to deal here with “many levels, all in an instant”.  The different level of brain biochemistry is essentially subliminal — but not for that reason hidden from fully conscious investigation. Indeed, it is open to investigation only as subliminal.

At roughly the same time that he began to reconceptualize the subliminal in this way as “implicitness”, McLuhan formulated the phrase (or the admonition): “the medium is the message”. And these were closely linked. The electric medium (“a new codification of experience collectively achieved by new work habits and inclusive collective awareness”) through which we have particle physics, genetics and modern medicine is inherently multi-level, simultaneous and therefore (combining these two) ‘all at once’. In many different sciences, this model of understanding — this medium, this “mode of intelligibility” — is taken for granted. Of course our bodies and brains work through biochemical interactions, and of course they do so simultaneously with our every action and thought, and of course this takes place on a different level from our conscious attention, and of course this does not mean that this molecular work cannot be investigated consciously but instead means that it can. But this is a new sense of the subliminal that is only a few centuries old (at least as a practical matter), which has in that time revolutionized every field into which it has been introduced.³

Now in the later 1950’s McLuhan proposed that this multilevel simultaneous electric medium of explanation be brought to bear also in the humanities and the social sciences. Further, he proposed that we initiate this transformation first of all by — Understanding Media.