This is a question I’ve begun to answer here and here. It’s certainly a metaphysical question, but one that I am inclined to think about in aesthetic terms. The short answer is this: an object is a conspiracy of sensations (or, if you prefer, qualities). For me, a quality is a sensation. It’s a mistake to see sensations as mental events. Sensations do not reside in the mind; they are external to it. An object, then, is what an empiricist might call a bundle of sensations, so long as we are clear that these sensations inhere in the extramental world.
From the phenomenological perspective, there is no need to posit a substance below the sensations. Objects appear to us and seem to enjoy a kind of independence from one another. This independence seems only phenomenal, however, because as soon as we begin to consider the relations in which the object is caught at any moment (physical laws, semiotic systems, affective attachments, etc.) we are led to what Tim Morton calls “the ecological thought.” Does that mean that objects reduce to ecological relations? Not at all. It just means that at the material level they never act alone; they always conspire with other objects. Whatever power they possess is an assembled, a ‘confederation’ in Jane Bennett‘s words. Consider how the object differentiates itself, or finds itself differentiated, from other objects at the aesthetic level. Levi’s blue mug would stand out from the red wall of my study not because from my perspective the wall forms a horizon upon which the blue mug is projected, but because blue and red (not to mention the wall’s texture and the mugs contours) contrast with one another and mark their differences, yielding a scene in which my sensorium is forced to acknowledge the existence of a mug, a wall, a scene, a room… It is not that the room and the mug are internally related, but that their manifestation is contingent upon a relation, some relation. This is not to say that their existence is relational, for the mug is never fully absorbed by any one of its relations–it withdraws from any particular relation, as Graham puts it.
But red and blue are the result of how light affect my eyes, right? Yes and no. The physical explanation of color calls for another account, one which considers the material conditions that ensure that our visual world is multicolored. This is neither a physical nor a phenomenological question, but a metaphysical one. When someone points out that the Indigo Bunting is not really blue, they are making a physical point (I take this example from Sartwell’s Six Names of Beauty). But it is also more than that. It indicates that there is a dark side of the visual realm, one which effects what we see without resembling what we see. It would be wrong, I think, to say that this dark side is devoid of aesthetic aspects. Indeed, it is the reality of the aesthetic. [Levi calls color an 'exo-quality', and I think I'm in agreement on this point.]
An object’s independence is asserted when it escapes absorption into any single relation, but it is also asserted when it effects some other object. An object’s identity/independence is determined by its effects. This is a point I borrow from Spinoza. A brick passing through a window enters into a relation with that window, causing that window to shatter. The brick may remain intact, the window not so much. The solidity of the brick and the fragility of the window, which are both aesthetic features–sensations–conspire at the moment of impact to create a singular event, even if not a singular thing. The window suffers more of a transformation than the brick, but neither’s reality is reduced by the event. Aesthetically speaking, the window is radically altered, but arguably not materially.
What is meant here by ‘materially’? Matter is perhaps nothing more than a conspiracy of qualities, or a set of sensations that unite for some time into a disposition. Fragility, then, is explicable at the molecular level, but also at the aesthetic level. Tap two crystal wine glasses together: you can hear their fragility. The difference between steel and wood can be described in aesthetic terms, how they feel, look, smell, etc., when a person encounters them with her senses. But we must also keep in mind that steel and wood interact with each other and manifest themselves relationally in ways that we cannot fathom, just as we do not know what it is like to be a brick at the moment that it impacts a window. Our ignorance does not entail that there is nothing to be said about the aesthetics of this event. Fragility is not in the eye of the beholder.
Working from the premise that objects interact not only physically, but aesthetically (consider the reflection at the top of this blog), along with the premise that sensations affect us and induce us to do things (see, for example, Saito’s Everyday Aesthetics or Johnson’s The Meaning of the Body), I would like to open speculation about the world of sensations untamed by the human sensorium. What would an object-oriented aesthetics look like? It would have to entail an account of the ecology of qualities, as well as an account of how humans find themselves animated by those qualities. A review of the history of sensation, impression, qualia, and so forth would have to be undertaken. Harman includes a chapter in Guerrilla Metaphysics on Levinas’s idea of ‘qualities without substance’. Levinas is a great resource thinking the independence of qualities, as is Deleuze’s book on Bacon.