Persistence Through Time
In virtue of what do material objects persist through time? This is not a difficult question to answer: the particles composing the object must persist through time (enough of them anyway) and they must stay spatially related to each other in the same way over time (to a sufficient degree). If you destroy all the particles, or disperse them to the wind, then the object ceases to exist. Normally objects cease to exist because of the latter circumstance not the former, since particles are pretty much indestructible: the object disintegrates and its erstwhile particles scatter abroad. This answer is not the same as the following answer: the successive stages of the object are similar and there is a causal connection between these stages. That answer says nothing about particles and their spatial relations. And it is vulnerable to counterexamples that don’t affect the first answer: what if all the particles are destroyed and a new set put in their stead—isn’t that a numerically distinct object that is just like the previous object? It doesn’t matter if the former state of the object causes the latter—we still have a numerically distinct (but qualitatively identical) object. There would be an appearance of identity over time, but the facts would belie the appearance. God (or an evil demon) could be doing this all the time, thus producing an illusion of persistence. Nor is similarity a necessary condition, since objects change a lot. You might say that the positions of the particles also change a lot over time, but we need not insist on exact preservation of position, so long as the particles don’t separate entirely and head off in different directions. The basic point is that the particles need to maintain a certain spatial cohesion. That, at any rate, is the rough idea of what the persistence of material objects consists in: particles sticking together. This is how we think of the persistence of such objects over time.
But when it comes to persons or selves or subjects or egos or souls this paradigm breaks down. For we cannot identify the particles that compose selves and we have no idea of their spatial relations. We can do this for the bodies of selves, so we know what we are talking about with respect to these entities; but the self isn’t the body, so we can’t just borrow its persistence conditions. The brain too persists in virtue of particle cohesion, but we have no idea what this could mean for the self: what are its particles and what kind of cohesion? The self isn’t a congeries of material particles (or immaterial ones). Its relation to the body and brain is unclear, so we can’t transfer an explanation of persistence from one to the other. The mind-body problem thus affects the problem of personal identity over time. Lacking this resource, people are apt to settle for the kind of theory I just rejected: personal persistence consists in qualitative similarity with (or without) causal connection. But this is intuitively too weak, and arguably also too strong. More important, it is the wrong kind of theory. It leads to notoriously problematic cases such as teletransportation: is the person who appears from nowhere on a distant planet really the person who entered the transportation booth or just someone very similar? If we could say that the particles of the person (not the body!) had all been destroyed or dispersed, then we could assert that the original person no longer exists (though a twin has appeared far away from where he or she was last seen); but that is precisely what we are not allowed to say, because we have no notion of what such particles (small parts) might be, or how they must be related over time. The paradigm of material object persistence doesn’t carry over and we have nothing to put in its place, so we are left fumbling in the dark. The mystery of the self (closely connected to the mystery of consciousness) prevents us from answering the persistence question in the standard way, and no other way suggests itself. In brief: personal persistence is a mystery. This is why we find ourselves so puzzled about whether we do persist over time, as we don’t find ourselves similarly puzzled for the case of material objects. It is the same table over time, because the parcel of particles has persisted, but am I the same person as yesterday? Am I perhaps just a duplicate self that appeared this morning when I woke up? There seems nothing to ground my identity over time. Maybe something does ground it, objectively speaking, but I am ignorant about what it is. The persistence of the conscious self is as mysterious as the consciousness that defines it, not surprisingly. Not that there is nothingmysterious about material object persistence, but personal persistence introduces another layer of mystery—it doesn’t conform to the relatively easy case of rocks and tables. Some sort of personal atomism would restore the parallel, but that seems like a remote dream in our current state of knowledge.
 We should allow for the case of organic entities that replace their particles every few years, but here we still have particle composition and particle position. It’s not like simply annihilating the particles and replacing them with a new particle configuration in the same place at some later time. Also, it is not at all clear that organic entities are really material objects given that their identity conditions are independent of the material objects that compose them (they are not aggregates of particles). They are closer to form than matter.
 I am thinking of Derek Parfit’s work here, but others have had similar ideas. What I say here also applies to survival without identity—we don’t know what it is for the self to survive. It can’t be the aggregate of particles that forms part of the brain because the self can’t be analyzed that way. At most this is a correlate of personal survival.