Between 1968 and 1972 McLuhan cited Heinrich Hertz (1857–1894) multiple times from The Principles of Mechanics (German original 1894, trans 1898):

The pain caused by the new media and new technologies tends very much to fall into the category of “referred pain”, such as skin trouble caused by the appendix or the heart. As with all new technologies, pain creates a special form of space, just a case again of [the] Heinrich Hertz law: “The consequences of the image will be the image of the consequences”. (War and Peace in the Global Village, 1968, 16)

As knowledge replaces experience in human affairs, senior businessmen feel a deep urge to go back to the campus. Having circulated around the world, and having immersed themselves in many problems, they now feel the need to specialize. That is, they are eager to do what the young detest, and the young are eager to do what the elders are fed up with. These inversions, or reversals, result from the exhaustion of the potential of any form, as Aristotle points out in the Physics. (…)  It is also known as the Hertz law: the consequences of the images will be the images of the consequences. (McLuhan to Pierre Elliot Trudeau, April 14, 1969, Letters 366)

the ancients attributed god-like status to all inventors since they alter human perception and self-awareness. Heinrich Hertz stated the same principle of complementarity and metamorphosis of our identity image in relation to technologies in his famous dictum: “The consequences of the images will be the image of the consequences”. It was Aquinas who alerted me (…) to the principle of complementarity inherent in all created forms. (…) Et ideo in toto tempore praecedenti, quo aliquid movetur ad unam formam, subest formae oppositae; et in ultimo instanti illius temporis, quod est primum instans [sequentis temporis, habet formam quae est terminus motus].¹ (McLuhan to Jacques Maritain, May 6, 1969, Letters 368-369)

When the secular man senses a new technology is offering a threat to his hard-won human image of self identity, he struggles to escape from this new pressure. When a community is threatened in its image of itself by rivals or neighbours, it goes to war. Any technology that weakens a conventional identity image creates a response of panic and rage which we call “war”. Heinrich Hertz, the inventor of radio², put the matter very briefly: “The consequence of the image will be the image of the consequences”. (McLuhan to Robert J. Leuver, July 30, 1969, Letters 387)

Ovid’s Metamorphoses follow the complementarity of Hertz’s dictum, “The consequences of the images are the images of the consequences,” illustrating “all growth as destruction”, as both “murdering and creating”. The technique of metamorphosis as a chemical change is by interface of two elements, or two situations. (From Cliché to Archetype, 1970, 50)

“Do-it-yourself” now permits use of the total environment as a private resource. Earlier, it had been an elite that exploited the “public benefits for private vices”. Now it is everybody who gets in on the act. This, naturally, via Hertz Law of Complementarity brings the flip or reversal of effect. Everybody becomes a bureaucrat in some branch of civil service, and this constitutes a police state (…) After increasing beyond some point, any service becomes a disservice. (Take Today, 1972, 82)

It is doubtful that McLuhan ever read anything by Hertz, even the ‘Introduction’ to The Principles of Mechanics from whose opening pages this supposed “Hertz law of Complementarity” was taken.  At a guess, at some point while he was at Fordham in 1967-1968 McLuhan heard or read some variety of the Hertz phrase “the consequents of the images must be the images of the consequents” (as the 1898 translation has it).  As seen in the passages above, McLuhan never quoted the translated phrase exactly and seldom cited it in the same way twice even in his own revised versions. However the case may have been in detail, the dictum must have struck him as a fitting formulation of the sort of “inclusive” relation, aka hendiadys, aka metamorphosis, aka synaesthesia³, that he took to be the structure of reality itself: “the principle of complementarity inherent in all created forms” (as he wrote to Jacques Maritain in the letter cited above). It is because reality is fundamentally structured in this dynamic and com/plicated way that, in McLuhan’s view (following Plato, Aristotle and many others) no one form can ever come to dominate all others. Instead, a currently dominant form must come to “exhaustion” at some point and give way to another.  It is this fundamental dynamic that displays itself as (and may therefore be said to ground) “inversions or reversals” — the “Hertz law of Complementarity”.

No single form can ever be more than a figure on this dynamic ground of forms — plural.

Exactly as fundamentally dynamic in this way, ground is itself necessarily subject to the complementarity that it everywhere enacts⁴: it inherently inverts or reverses itself to figure or cliché.  It follows that the latter are the “inexact” sign of the former.  (Modernity is that time when the signification, or the significance, of the figure or cliché becomes obscured. We lose sight of its sense. And this because we have first of all lost sight of the grounding sensus communis — that which is always beyond any one sense and is always more than any one sense.)

The way forward to McLuhan’s fourfold “Laws of Media” (the interplay of two sets of “inversions or reversals”) was close at hand.  A couple years after his spate of Hertz citations, in February 1974, he would close a lecture at the University of South Florida (‘Living at the Speed of Light’, UMe, 226-243) as follows:

The laws of the media (…) are quite simply that every medium exaggerates some function. (…) They obsolesce another function; they retrieve a much older function; and they flip into the opposite form. The [best example]⁵ I know to illustrate this principle, which [principle] works for all media whether it’s a teaspoon, a corset or a motorcar, is money. Money increases transactions; it obsolesces barter; it retrieves potlatch or conspicuous waste; and it flips into credit cards, which are not money at all. Now every medium starts off by exaggerating something that we all have before finally flipping into the opposite of itself. (…) The laws of the media (…) are at least a hope that we can reduce (…) confusion to some sort of order.

At this time in the early 1970s McLuhan was attempting to specify “the life of the forms and their surprising modalities” (McLuhan to Joe Keogh, July 6, 1972, Letters 412). In a sense, this was, of course, what he had always been up to. But now, given his dire health outlook and his palpably declining audience, he was particularly concerned to focus his message. The “Laws of Media” represented his hope to order the confusion and despair that modernity finds in “all growth as destruction”.

Hence his citation at this time of his friend William Wimsatt from Hateful Contraries (1965) in From Cliché to Archetype:

the English Augustans were, at their best and at their most characteristic, laughing poets of a heightened unreality. The world which the Augustan wit found most amusing and into which he had his deepest visions was an inverted, chaotic reality, the unreality of the “uncreating word”, the “true No-meaning” which “puzzles more than Wit”. The peculiar feat of the Augustan poet was the art of teasing unreality with the redeeming force of wit — of casting upon a welter of unreal materials a light of order… (109)