"...rhetoric is really the art of the background, and if philosophy is not the science of the background then it is nothing."
- Graham Harman
"Rhetoric is not the devious art of non-rational persuasion, but the best tool we have for exposing the unstated assumptions that lie behind any surface proposition. The analytic contempt for rhetoric and metaphor must not be emulated - not just because this attitude leads to boring results, but because it is philosophically false."
- Graham Harman
Speaking against what he calls Analytic philosophy's "knockdown arguments" that have yet to produce a Golden Age of philosophy, Graham Harman tells us that he upholds instead "hyperbolic readings of philosophers against critical ones, since critique assumes that the major problem with any piece of writing are the logical errors it contains. By contrast, to hyperbolically imagine the complete victory of any philosophy is to simulate a social environment in which it is widely held to be free of logical blunders, and hence this method allows us to focus on what Whitehead calls the 'coherence and adequacy' of that philosophy" (PN: 175). 
He goes on to tell us that he holds to the idea that a 'good rhetoric' is the key to philosophy (PN: 176): for the simple reason, that "rhetoric deals with veiled background assumptions rather than explicit dialectical figures - and if philosophy does not expose background assumptions and play counterpoint against them, then I do not know what philosophy is for (PN: 176). Instead of the logically precise yet boring precision of Analytic philosophy he admires the empowering breadth and vastness of thought within such philosophers as Plato, Spinoza, and Leibniz who "do not make fewer logical blunders than the average university professor, but are simply much vaster in adequacy, coherence, originality, relevance, and insight (PN: 176 emphais mine).
That Marshall McLuhan's work on the trivium and tetrads left a lasting impression on Harman might be a clue to his fascination with rhetoric:
"In Laws of Media, Marshall & Eric McLuhan introduce their concept of the tetrad. Every medium can be described in terms of four polarities: enhancement, obsolescence, retrieval, and reversal. In his preface to the work, Eric McLuhan boldly describes the tetrad as “the single biggest intellectual discovery not only of our time, but of at least the last couple of centuries.” Recently, he stated that he “does not retract one iota” of that brazen claim. But not only has this assertion not been accepted – it has rarely even been mocked. The tetrad has largely been ignored, even by admirers of the McLuhans. This talk will proceed under the assumption that the tetrad is, in fact, the greatest intellectual discovery of at least the last couple of centuries."
- from The Greatness of McLuhan by Graham Harman
McLuhan cites a passage from Cicero's De Oratore depicting the doctus orator, the ideal philosopher and citizen:
"Whatever the theme, from whatever art of whatever branch of knowledge it be taken, the orator, just as if he had got up the case for a client, will state it better and more gracefully than the actual discoverer and specialist."
The idea of encyclopedic knowledge was an aspect of this whole practice, and yet it was open to all for as McLuhan says, it is "not surprising, therefore, that Cicero should hold that philosophy is something that anybody can easily learn, since, like the Stoics, he held that the principles of philosophy of wisdom are innate in the hearts of all men...and that unless a man can learn a thing quickly he can never learn it at all." 
Encyclopedic knowledge is a preparation for the creation of the true orator, whose prudence-or decorum-fashions his eloquence to make it identical to his wisdom. The driving aspiration of ancient rhetoric lay in this identification of internal wisdom with external delivery. From a 1946 essay, McLuhan condenses his pursuit of that theme in the Nashe thesis:
The origin of this important claim for the inseparable character of eloquence and wisdom would seem to lie in the familiar doctrine of the Logos, which may be supposed to have arisen with Heraclitus. Society is a mirror or speculum of the Logos, as, indeed, are the external world, the mind of man and, above all, human speech. Society, ideally the cosmopolis or perfect state, claimed the devotion of every virtuous man. And just as Zeno considered wisdom or prudence "not only as the first of the virtues, but as the foundation of all," so political prudence is the noblest sphere in which to exercise this virtue.
In classical rhetoric there are five divisions of labour: invention, arrangement, expression or style, memory and delivery. Perhaps nowhere so vividly as in these categories, are wisdom and eloquence one. But the categories must be understood analogically. Since the sixteenth century-it is one of the legacies inherited from Ramus-our knowledge of invention and arrangement (inventio and dispositio) have been governed and shaped by a schooling of logic. "Expression," "memory" and "delivery" seem no more important than did the high school course in public speaking where we trained at them. But considered analogically, these categories become, as McLuhan would say in 1975, "nothing else than the five mental faculties of man, perceived comprehensively." Inventio, dispositio, elocutio, memoria and pronunciato are inherently not so much disciplines for the orator as they are the very body of human wisdom and decorum.
He concludes the Nashe thesis on a note evocative of so many weary but elated passages in the diaries:
"And so, I interrupt what I hope to be able to conclude another day. It is impossible not to be conscious of the many defects of this study; but it is, likewise, impossible to have surveyed the territory in question without acquiring a vivid sense of its largely unexplored character."
Graham Harman tells us in an interview that he learned of Marshall McLuhan from someone in Chicago at a time when he was interested in Jean Baudrillard: "I once heard someone say “Baudrillard stole all his ideas from McLuhan,” and though my first instinct was to defend Baudrillard, this made me think that I should read McLuhan too." He goes on to describe his readings of McLuhan and "being immediately drawn to the tetrads, "which even some McLuhanites still don’t appreciate." Then he makes McLuhan's influence on his own work, saying,
"Not surprisingly, what I most like about Marshall McLuhan are two factors that are crucial for my own work: (1) his respect for the power and efficacy of individual things, and hence his refusal to put the human subject at the center of everything as in most modern philosophy; (2) his respect for the power of the formative background over the surface figure, and his resulting greater interest in rhetoric than in dialectic. Rhetoric still has a bad name: it’s “mere rhetoric,” you know. But rhetoric is really the art of the background, and if philosophy is not the science of the background then it is nothing. (my emphasis) Even Socrates, the supposed champion of explicit dialectic, is actually a rhetorician insofar as he thinks we must know what virtue or friendship is before we know what its qualities are. And given that any explicit statement about anything means enumerating a list of its qualities, Socrates is pointing us toward a deeper reality that dialectic cannot reach. Aristotle, too, puts enthymemes not only at the center of his Rhetoric and Poetics, but possibly at the center of his entire philosophy— if, like me, you interpret primary substance as that which is not just a material basis for qualities, but as something deeper than all qualities. ...
Rhetoric is not “mere rhetoric”: it was half a day’s instruction in the Lyceum, and Aristotle wasn’t just teaching rhetoric for “regrettable practical reasons” such as that “we live in an imperfect, irrational world.” No: in some ways the background medium (in McLuhan’s sense) really is the subject of all philosophy. Heidegger even calls it Being."
1. Graham Harman, Prince of Networks: Bruno Latour and Metaphysics (PN) (re.press Melbourne 2009 )
2. Bill Kuhns, The War Within the word: Mcuhan's History of the Trivium (McLuhan Studies 2010)