Harman's central philosophical thesis is one of those ideas that is initially baffling and then, once you get it, hard to unsee. Here it is:
Every object withdraws from every relation it enters. No description, no interaction, no measurement, no contact with any other object ever exhausts what the object is. Objects have a real being that exceeds their appearances, their effects, and their relations. This is not a mystical claim, it is a structural one about the nature of objects.
Harman starts from a reading of Heidegger's famous analysis of tools. When you use a hammer, Heidegger says, you don't experience it as a thing with properties, you experience it through its function, transparently, as an extension of your intention. You become aware of the hammer as a hammer only when it breaks: when it stops functioning and becomes present-at-hand, an object with properties, rather than ready-to-hand. Heidegger uses this to make a point about human existence. Harman uses it to make a point about all objects.
His argument: even when objects relate to each other, not just when humans interact with them but when a flame touches cotton, when gravity pulls a moon, when a virus infects a cell, the relation does not capture everything about either object. The flame "translates" the cotton into its own terms (inflammability, combustibility) but this translation is always partial. The cotton has properties that the flame never encounters. Objects interact through what Harman calls sensual profiles, simplified, partial translations of each other, while their real nature withdraws.
This gives Harman his quadruple object: every object has two dimensions along two axes.
The first axis: real vs. sensual. The real object is the object as it is in itself, withdrawn from all relations. The sensual object is the object as it appears in relation to another object. You never encounter the real object directly, only ever sensual profiles of it.
The second axis: object vs. qualities. Objects are unified wholes that are more than the sum of their qualities. The qualities of an object can change without the object ceasing to be the same object (the apple is now red and hard, now brown and soft, it is still the apple).
Combining these two axes gives four poles: real objects (withdrawn things in themselves), sensual objects (objects as they appear in relations), real qualities (the genuine hidden profile of an object), and sensual qualities (the properties as encountered in relation). Harman argues that all philosophical problems are ultimately about the tensions and translations between these four poles.
This framework has generated enormous controversy. Critics argue that the "withdrawal" claim is unfalsifiable and does no philosophical work, it just asserts a hidden dimension that is by definition inaccessible and then builds theory on it. Harman's defense: the withdrawal claim is a logical consequence of the argument that no relation exhausts the relata, which is demonstrable in every case of translation and interaction. The hammer does not "know" everything about the nail; the flame does not "know" everything about the cotton. Withdrawal is not a mystical supplement but the logical implication of taking the gap between any relation and its terms seriously.