Graham Harman, commenting on Jane Bennett’s Vibrant Matter:
It is certainly true that context and relation can affect the reality of an individual thing. It does not follow that each last detail of a context or relation changes the thing that is involved with them. An individual is a kind of filter (or “firewall,†as I often call it) responding to some relation partners but not all. To be affected by something outside us is a special case, even if not a rare one. Countless things happen around us without this entailing that our reality registers each tiny fluctuation in such a way that it changes who we are.
This is the closest I have come to agreeing with Harman about objects as individuals. I still want to argue for promiscuous interrelations among objects, rather than seeing them all as vacuum-sealed; but here, my only qualification would be that I think that every entity makes a “decision,” as Whitehead puts it, as to which “relation partners” (Harman’s phrase, not Whitehead’s) it responds to, and which it ignores. In Whitehead’s parlance, this ignoring another entity could take the form either of what he calls a “negative prehension” (which is a decided refusal) or of the fact that the other entity has only a “negligible” influence on the entity that is making a decision. So, while I think that “to be affected by something outside us” is the general case, rather than a special one, in practice the degree to which an entity is affected is fairly minimal.
Harman further remarks that “Unless a philosophy can account adequately for the fact that not all changes make a difference, then its sense of individuals is too weak.” And again, I mostly agree. But I would argue that this condition is met by Whitehead’s claim that, although in principle an entity is affected by all the other entities in the universe (or at least in its light cone), in many cases  (and probably in the overwhelming majority of cases), this influence is negligible.
I still differ with Harman in thinking, following Whitehead (who in this case is himself following William James), that the existence of an entity is punctual, and that the endurance of an object through time needs to be understood as a succession of entities, with a large measure of inheritance accounting for the continuity. This is why (as I said at the OOO conference last week — but this part of my talk still needs some revision) the question of whether an entity remains “the same” over time is a relative one, a matter of degree.