Both Innis and McLuhan recurred repeatedly to Plato’s description in the Phaedrus of the invention of letters by the Egyptian god Theuth.  Plato discusses Theuth’s invention also in the Philebus…

Some god or divine man, who in the Egyptian legend is said to have been Theuth, observing that the human voice was infinite, first distinguished in this infinity a certain number of vowels, and then other letters which had sound, but were not pure vowels; these too exist in a definite number; and lastly, he distinguished a third class of letters which we now call mutes, without voice and without sound, and divided these, and likewise the two other classes of vowels and semivowels, into the individual sounds, told the number of them, and gave to each and all of them the name of letters; and observing that none of us could learn any one of them and not learn them all, and in consideration of this common bond which in a manner united them, he assigned to them all a single art, and this he called the art of grammar or letters. 
(….) 
And the precise question to which the previous discussion desires an answer is, how they are one and also many, and are not at once infinite, and what number of species is to be assigned to either of them before they pass into infinity.
(….)
if we are not able to tell the kinds of everything that has unity, likeness, sameness, or their opposites, none of us will be of the smallest use in any enquiry. (Philebus 18, Jowett trans) 

Plato’s discussion here relates back to Heraclitus: you cannot step into the same river twice. That is, the same river — “this common bond which in a manner united them” — is by its nature both “one and also many” such that when we encounter it we do so only (only!) in a particular and limited manner (cf, “a certain number”, “a definite number”, some discrete “one of them”).  But this encounter with some particular “one” has the further implication that “none of us could learn any one of them and not learn them all”. For when a child first understands a word as a word and not merely as a meaningless sound, it has thereby learned not just this particular instance of language but what language is per se.

Although the same river is always different, stepping into it is to experience — the river.

Now language and grammar are a kind of water in which humans qua humans swim: “this common bond which in a manner united them”.  “United them”: both in their particular conversation and in their common humanity.

McLuhan famously observed  (apparently following John Culkin, who, in turn, may have been following Einstein?) that

We don’t know who discovered water but we are pretty sure it wasn’t a fish! We are all in this position, being surrounded by some environment or element that blinds us totally; the message of the fish theme is a very important one, and just how to get through to people that way is quite a problem. (Contribution to Technology and World Trade, 29, 1966)

You know there is an old saying (not so very old) : “We don’t know who discovered water, but we are sure it wasn’t a fish.” That is literally true. It is inconceivable that a fish could discover water or that anybody could discover anything that was totally surrounding his senses. That is the one thing you will never know. (‘Education in the Electric Age’, 1967)¹

we don’t know who discovered water, but we’re pretty sure it wasn’t a fish. The one thing you can never see is the element in which you move. (‘Canada, The Borderland Case’, 1967, in Understanding Me, 106).

we live invested with an electric information environment that is quite as imperceptible to us as water is to a fish. (…)  The fish knows nothing of water” (Counterblast, 1969, 5 and 75).

McLuhan may be read as raising, once again, the question that is at least as old as Heraclitus and Plato: How to communicate about the medium of communication in which humans exist as fish do in water?  

After 2500 years of failure, in a global village with nuclear weapons, he was of course quite correct that “just how to get through to people that way is quite a problem”.