One of the sources of philosophical confusion in talking about self-continuity [e.g., "I'm not the same person I used to be"] is that selfhood and personhood are not quite the same thing. And both are ambiguous (though “personhood” more so). They both incorporate subjective and objective criteria, “internal” and “external” criteria; but the idea of the self nevertheless relates to one’s subjectivity more closely than the idea of the “person” does. For the self is just one’s sense of self, one’s self-consciousness. Consider someone who undergoes electroshock therapy that erases his memories and changes his personality. Because he doesn’t recognize himself in “his” past, we are willing to say that he now has, or is, a different self than before. He has a new self-identity. But is he a different “person”? From one perspective, yes: there are major psychological discontinuities between his past and his present. He has changed; “he’s a different person,” as we colloquially say of someone who acts very differently than he used to. But from another perspective, the answer is no: there are physical continuities and even some mental continuities between his past and his present. So we say, for example, that Robert Pirsig (the author of Zen and the Art of Motorcycle Maintenance) experienced electroshock therapy and drastically changed as a result of it; i.e., it is one person who was depressed, then had electroshock therapy, then emerged very different from before. Pirsig, however, would deny that he is the same person as before the therapy—he even gives the latter a different name, “Phaedrus”—because, from the “inside” (of his consciousness), they seem to have nothing in common. They’re different people. From the “outside,” though, we see his physical continuity and so forth, and we judge that he and Phaedrus are one person whose peculiar psychological history has split him up, “for himself,” into two people, or rather two selves, two identities.
The basic meaninglessness of the idea of a person, or rather of “personal identity,” is responsible for many philosophical headaches. Consider a thought-experiment from Derek Parfit. Suppose a teleporter works by disintegrating your original body and reconstituting it in exactly the same form in another location. Your copy is identical to you in every respect, shares your memories, your personality, the precise configuration of the cells in your body, etc. What happens, then, is simply that you step into the teleporter and suddenly appear in a different location. As far as you’re concerned, it is you who appears, not just a copy of you, because you remember being in the teleporter a moment ago. But imagine that one day the machine doesn’t work as it’s supposed to: instead of disintegrating and appearing in a new location, you remain standing in the teleporter. A moment later you see on a video screen a copy of you at the new location, where “you” have been teleported. But this time, obviously, it isn’t you; you are still standing in the machine. It’s just a copy of you. On the other hand, as far as the copy is concerned, he is you: he remembers being you, etc. For him, nothing has changed from previous teleporting experiences. So it seems that, in a sense, there are now two of you. But that’s odd, even nonsensical. It’s also odd that intuitively we seem to think that when the teleporter was working properly it was you who appeared in the new location, whereas later it was only a clone of you. In both cases, after all, what happened is just that a copy was made of you and transported somewhere else.
Such puzzles and conflicting intuitions result from the contradictory and confused nature of the idea of personal identity. It incorporates both subjective and objective criteria, and both psychological and physical criteria. That is, it has no determinate meaning; its uses are governed by “family resemblances,” as Wittgenstein might say. The artificiality and superficiality of the concept—its merely constructed nature, its “non-naturalistic,” “unreal” character—is shown by the fact that in the above thought-experiment, the teleported copy both is and is not you. The copy evinces psychological continuities but no direct physical continuities. And even the psychological continuities are problematic: in the second scenario, the copy is quite sure that he has the same consciousness and is the same self, the same person, he was a moment ago when standing in the teleporter, but the original you (still standing in the machine) would certainly take issue with that.
We can’t dispense with the notions of personal identity and self-continuity, since they are conditions of our experience of ourselves and others; we can, however, intellectually recognize their incoherence. In ordinary contexts, as opposed to thought-experiments, the criteria of self-continuity are physical continuity and memory connections. I look at a picture of myself taken twenty years ago and say, “That’s me when I was 12.” In other words, that 12-year-old became this 32-year-old, and the latter used to be the former. There are physical and psychological continuities between the two that justify my saying we are the same person. But when you look more carefully at these criteria you see that they break down in certain “borderline” contexts, and thus that the concepts of self-continuity or a personal identity that extends through time are not well-formed (in the way that, say, ‘bachelor’ is). They are socially necessary fictions, fuzzy around their edges. Strictly speaking, even to say that the 12-year-old in the picture is me is not wholly meaningful or correct. (There are, after all, very significant differences between us.) It is merely an effective shorthand for saying that that body and consciousness evolved into this body and consciousness, and that in some sense I recognize myself in that earlier self. -The idea of a self-substance that extends across time has no place in a scientific account of the world.
 Parfit, Reasons and Persons, pp. 199, 200.
 I.e., he isn’t a mere “clone” but the real thing.