In his essay Intentional Objects for Non-Humans (2008 via Anthem) Harman tells us that while "Husserl’s phenomenology describes things in terms of their appearance to consciousness, Heidegger notes that things primarily do not appear in consciousness. Instead, they withdraw from view into invisible usefulness. The floor in this room, the oxygen in the air, the heart and kidneys that keep us alive, are generally hidden unless and until they malfunction."
Recently I came across this unique site Thinking with Shakespeare by Julia Reinhard Lupton as I was doing some research on that great poet/playwright's work for another community blog I work with dealing with purely literary pursuits. Her essay suddenly gave me the uncanny feeling that I was overhearing Graham Harman's voice popping out of a black box like a found object amid her strange abstract for a talk on Romeo and Juliet:
"Things disappear: they disappear into the routines of use in daily life, except under circumstances of failure, want, or breakage, or in moments of special appreciation and acknowledgment, which include both curation and celebration. Things also disappear in academic analysis, insofar as they too quickly turn into symbols or symptoms, semiotic messages divested of their lived properties. By taking things as objects of analysis, we tend to make their normative modes of being withdraw from attention. I would like to grasp here the special ways that things disappear and reappear in daily life, in order to short-circuit the way that things disappear in academic discourse."
I would like to grasp instead just that strange moment when an object decides to withdraw or recede into that sphere of "invisible usefulness" that has nothing to do with our attention. Harman in his essay tells us that the important thing to distinguish is not that an object is either invisible or visible, but "of the transformation of a thing’s reality." Against those that would give a pragmatist reading to Heidegger's Tool-analysis he tells us that "praxis is more stupid than theory. This is why theory was invented, after all. The act of using something distorts and oversimplifies its reality even more than theory does. Heidegger is no pragmatist."
Yet, Harman, tells us like the good revisionist he is, that Heidegger did not get it all, he needed to go further to complete this line of thought that would have built an "unexpected highway from Heidegger to pansychism," one that acknowledges the subtle truth that if "theory and praxis both distort, caricature, or transform the hidden reality of things, then the same must be true of any relation whatever." The key to this is that it doesn't matter if humans or non-humans relate to objects, the key is relationality itself; for only through relations do objects emerge: or, as he states it: "all relations are on the same footing."
Ultimately Graham Harman's basic model of Object-Oriented Ontology is based on the unique premise that objects are irreducible to any relations with other objects, and that while dependent on their component pieces, they are also irreducible to those pieces; objects merely encounter caricatures of each other, and in ontological terms this happens in the same way whether a human or animal is involved or not, the fact that objects cannot encounter each other directly means that only indirect relation is possible. However, we cannot follow either the occasionalists (who make God the sole medium of relation) or Hume or Kant (who invert occasionalism by making human habit or categories the only known site where this occurs), there is an infinite regress of objects, but not an infinite progress toward ever larger ones, up to and including the “universe as a whole” (which doesn’t really exist for me); both real and sensual objects are polarized between an object-pole and a quality-pole, this yields a fourfold structure, which generates not only space and time, but their previously disowned sisters which we might term essence and eidos. 
In sliming down the above rendition of his model Harman tells us that "you’re going to have a hard time not only finding a past or present philosophy that agrees on all of these points (perhaps that’s setting the bar of resemblance too high) but even of finding one that matches the basic model: a cosmos of objects at countless different scales, all real regardless of whether anyone or anything is currently interacting with them, and all withdrawing from one another and interacting only indirectly through a sensual medium."
Levi Bryant on Invisible Objects
Levi gives us his rendition of George Molnar's Powers: A Study in Metaphysics. He took the lead from Steve Shaviro's advice from Pinnocchio Theory in which he tells us that Molnar "asserts a realist ontology, one that is directed against the skeptical empiricism of the whole tradition derived from Hume." He goes on the draw parallels between Molnar and Harman, saying, "Molnar insists, as much as Graham Harman does, that a thing, or an object, is not just a bundle of properties or characteristics, but exists in its own right apart from and in addition to these."
Levi on this advice draws parallels between Molnar's use of powers and his own conceptual framework of onticology: "I argue that objects are split or divided between their virtual proper being and their local manifestations. The virtual proper being of an object is its powers, what the object can do, while local manifestation is the properties that an object comes to embody or actualize." Instead of Harman's use of the split between the real and intentional object, Levi gives us a new set of metaphors: virtual proper being and their local manifestations. He equates his concept of VPB (virtual proper being) = powers and the LM (local manifestation) = properties.
Levi tells us that Molnar breaks these powers down into five feature sets:
1. Directedness, or physical intentionality:
"Intentionality is not restricted to the domain of the mental, according to Molnar, but is a feature of physical objects as well. The directedness of a power is a form of intentionality insofar as a power is directed towards its manifestation in a quality. Thus, for example, the solubility of salt is directed towards salt dissolving itself in a liquid. The manifestation is that towards which the power (solubility) tends or is directed."
"The key feature of powers is that they are independent of their manifestations. Salt has the power of solubility even if it is never dissolved in water. In this regard, powers are non-identical to their manifestations. This is one of the reasons that I endlessly emphasize the role played by regimes of attraction in the actualization of objects. Regimes of attraction can be roughly equated with context. Insofar as powers are independent of their manifestations we never entirely know– to quote Spinoza and Deleuze –what an object can do. We discover the powers of an object by placing it in different contexts and seeing what it does. Yet in doing so, other powers contained within objects remain dormant insofar as they can only be actualized or manifested in other contexts."
"By this Molnar means that powers are not mere possibilities, but are real features of objects. They belong to the actual object itself."
"...powers are non-detachable “parts” of objects. It is for this reason that I’ve been led to equate power or virtual proper being with the substantiality of objects.
5. Objective features:
"...powers are objective features of objects. Hume had argued that our notion of powers is merely a psychological effect of how the mind associates events. By contrast, Molnar argues that powers are real properties of objects."
Levi seems particularly fascinated by the second feature, independence, saying, "I find the feature of independence particularly fascinating. If the substantiality of objects is defined by their powers and powers are independent of their manifestations in qualities, this seems to entail that it’s possible for there to be objects that are completely unmanifested or “invisible” within the world. This would take place in the case of objects whose powers are actual and real, but which are completely dormant. Such objects would appear as if they don’t exist precisely because they don’t appear at all, but would nonetheless be entirely real and existent."
But why do these objects need to be invisible at all? Why not transparent? Think of the ocean within which the fish live and breath without ever thinking of the medium within which that move and swim; is it not a sort of sleeping object through which they move but in a way that is transparent rather than invisible to them as an independent object? For us the ocean can become a force for destruction as all those shipwrecks like the Titanic will attest too. But the ocean for us as compared to the creatures that live within its depths is another level or scale within that ontological region of our universe that is open to us only through a negotiated struggle between levels of existence (i.e., we must have dive suits or submarines to plunge deep into its depths), except as a vehicle for our own tools to cross (ships), plunder (fishing), dig (oil derricks), or study (climatology).
To be "unmanifested" does not imply appearance (invisibility/visibility) at all. As Harman stated in his essay above the important thing to distinguish is not that an object is either invisible or visible, but "of the transformation of a thing’s reality." In this statement Levi seems to confuse the issue of the real and intentional object by implying that if an object is dormant it would appear as if it didn't exist precisely because it doesn't appear at all, yet would still be "real and existent". But again what does appearance have to do with it? Since a real object never appears to us or another object directly, but only indirectly through its intentional or sensuous appendages, properties, or qualities then why does appearance become a problem? An object does not need to appear to us or another object to relate, think of all those little microbes and bacteria that daily infest our lives and travel in the midst of that transparent region of our terrestrial globe we call the atmosphere, that negotiate there way burrowing through our porous flesh and into our subdermal systems and into our bloodstreams infesting us with all those terror born diseases we term epidemics.
Harman speaks of a "carnival of levels extending throughout the cosmos" in which objects flicker from one level to another using a common language of charm or brute force in which they are able to persuade or annihilate one another. Each level of the world has its own unique language by which objects communicate with each other, and it is this "intermediary zone through which objects signal to one another, and transfer energies for the benefit or destruction of one another that the carnal phenomenology of Object-Oriented philosophy touches base with. For it is in this intermediary zone of the carnal and sensual medium that "objects are able to interfere with one another" (GM: 70). It is in this interzone where "beings collide with one another in a field, in a series of levels that connect them with one another. These objects can never be fully deployed in any single level, since their nature is never to manifest themselves entirely in any interaction at all" (GM: 70). 
The appearance of an object either to us or another object will always be partial or perspectival, for the simple reason that it reveals itself only through its intentional properties or features and does that through a negotiation and translation that is a distortion of the real objects hidden interior life. A real object is autonomous which always-already is independent and withdrawn, not fully manifested in any relationship of any type except through the interaction in that field or volcanic core where all change manifests itself. We do not need to worry over appearances, objects can be real and independent, withdrawn and sleeping and dormant without being either invisible or transparent. Objects can exist without relations; yet, as Harman has reiterated, it is only in relation that objects perceive.
I will hold off on discussing the issues of language, metaphor, hyperbole, etc. for now, except to ask why we need so many strange obfuscations of terms that seem to distort rather than clarify. By this I am referring to Levi's use of virtual proper being and local manifestation as against Harman's real and intentional objects. I see that there is a difference in emphasis here, that VPB seems to imply a potentiality; ergo., the reason for the need in equating Levi's terms to Molner's powers and properties. Maybe I'm all we here and that this is less an emphasis on language, than it is on philosophical influence: knowing that Levi came out of struggles with Deleuze, and Harman out of Husserl/Heidegger. The love of one's originals is always a driving force in one's ongoing projects, and always difficult to overcome or subtend in a way that is beneficial to all concerned. This is part of being individuals.
I am still an avid reader of both philosophers, and it is interesting to see this friendly struggle or agon within the OOO community. I await, avidly, both Harman's (On Meillassoux, The Quadruple Object, etc.) and Levi's (The Democracy of Objects) new books. Hopefully the deeper implications of both philosopher's systems will be detailed out and further explicated. I see in OOO something congenial to my own thinking and wish them both the best in coming years as they continue to develop productive and worthwhile lives and thought.
1. now Peirce got there first - from Graham Harman's blog Object-Oriented Philosophy
2. Guerilla Metaphysics: Phenomenology and the Carpentry of Things (GM) (2005 by Carus Publishing Company)