by Massimo PigliucciAnyone who knows me also knows that in the past few years I got inescapably hooked on the British scifi series Doctor Who , though I’ve actually watched only the reboot version, which started in 2005 and is now in the 6th season. It is actually the longest (though not continuously) running scifi show ever , having started on 23 November 1963 (at 5:16:20pm, GMT, to be precise — time matters for a show based on time travel).For those unfortunate souls who know nothing about it, the Doctor (as he is known on the show) is (almost, as it turns out) the last of an ancient race known as the Time Lords, and he jumps back and forth throughout space-time by means of a device called the TARDIS (which stands for Time And Relative Dimension In Space. It’s bigger on the inside! And it looks like an old fashioned police box, as a disguise). He has all sorts of fantastic adventures, often involving one or two human companions who come along for the ride. And running, lots of running.Naturally, I am currently reviewing a collection of essays of philosophy for the public called Doctor Who and Philosophy : Bigger on the Inside, published by Open Court and edited by Courtland Lewis and Paula Smithka. But this isn’t the review of the book (which will eventually appear in Philosophy Now ). Instead, I’m going to focus on one of the chapters, by Michael Hand, entitled “Regeneration and resurrection,” where the author takes on the logical possibility of resurrection in an afterlife, using it as a springboard to talk about the thorny metaphysical issue of personal identity (there are several other chapters in the book devoted to personal identity, by Patrick Stokes, Greg Littmann, Richard Hanley, and David Kyle Johnson, but I won’t comment on those).Before we get into the thick of it, you need to know that the Doctor has an interesting characteristic: from time to time, he “regenerates.” Regeneration is something that Time Lords can do a finite number of times (twelve, to be precise), and which they undergo when their current incarnation is mortally wounded (which has happened a number of times over the almost half century of the show). They emerge from the process with some interesting characteristics: first, their physical appearance is different, and dramatically so; second, their personality is more or less slightly altered; third, they retain their memories of past incarnations. The current Doctor, played by Matt Smith , is the 11th such incarnation. (My favorite Doctor so far is David Tennant , the 10th incarnation, though I certainly appreciated Christopher Eccleston , number nine.)It is by examining the concept behind regeneration that Hand begins his investigation of the logic of resurrection (of which Time Lord regeneration seems to be a subtype) and more broadly the metaphysics of personal identity.The first question asked by Hand is whether resurrection is logically possible. This is obviously not the same as asking whether it is physically possible, since plenty of things do not contravene logic while still being impossible in our particular physical universe. Asking about the logic of a concept is, in part, to ask whether the concept is coherent. Prima facie, resurrection faces two possible logical objections, according to Hand: one is related to personality changes after resurrection (or, in the Doctor’s case, regeneration); the other to the physical constitution of the resurrected body.According to Christian lore, for instance, when the good guys are going to be resurrected at the end time they will get to live in a paradise where nobody does nasty things to anyone else and all is bliss. Setting aside the sheer boredom of such a place (just compare Dante’s Paradiso with his much more captivating Inferno to appreciate my point), this presents a problem. As Hand points out, the resuscitated people will have to have undergone a significant change in personality to make eternal bliss work, because it is simply not human nature to get along perfectly with everyone else, for eternity. But if, say, our editor Phil makes it to paradise (he won’t, he’s an atheist, even though he is a nice guy), and has to undergo a significant personality change in order to get there, then in what sense will it still be the Phil I know? For instance, if certain versions of the Christian “eternal bliss” are correct, there won’t be martinis to be found anywhere in the joint, and I simply can’t imagine Phil-now being able to stand such place, much less to agree that it is a “paradise” in any meaningful sense of the word.The second problem Hand identifies is rooted in the Christian idea that the resurrection will involve a reconstitution of one’s physical body (just like the Doctor’s regeneration). The problem is: where will all the necessary atoms come from? You see, the way the biosphere works, most of the atoms in our bodies are recycled, having been through countless other bodies before (mostly not of fellow humans, but possibly some did come from other members of Homo sapiens). When God will reassemble everyone, what happens to all those people (presumably a large number) who were made of partly overlapping sets of atoms? Well, you may say, but God isn’t bound to use the exact same atoms, he could build replicas of the bodies using spare atoms taken from somewhere else. Indeed, but then we wouldn’t have the same bodies, and there seems to be a strong sense in which we are who we are in part because of our particular physicality. What gives?In thinking along these lines, we have just touched on two of a number of ideas about what constitutes personal identity: continuity of personality and bodily continuity. But Hand points out that neither of these are knock down arguments against the logical possibility of resurrection (or, more importantly, the logical consistency of Doctor Who episodes!). Fans of the show seem to have no trouble “recognizing” the Doctor from one incarnation to the other, regardless of the fact that his personality is measurably different, and also that his body is clearly not the same. These judgments are shared by most people in real life: we know of plenty of instances in which an accident, or a disease, dramatically alters someone’s personality, and yet we say that he is “not the same” only in a metaphorical and somewhat poetic sense. Moreover, our bodies are certainly not made of the same atoms as the bodies we had when we were younger (and we surely don’t look the same either, if enough time has passed!), and yet — again — we don’t say that we and our younger selves are different persons.A third major concept of personal identity (other than character and physicality) is based on memory: we are a given person because we have certain memories. Here, however, Hand confronts us with an episode of Doctor Who ( Journey’s End , part of the 2008 season, with David Tennant playing the leading role) whereby an accident gives us a replica of the Doctor. The replica is, obviously, made of different atoms, and displays a slightly different personality from the original. But his memories (and accompanying emotions) are exactly the same as those of the “real” Doctor. Indeed, one of the Doctor’s companions, Rose Tyler, objects that the replica is not, in an important sense, the Doctor. Most people would, I think, agree that Rose is right, which throws a huge monkey wrench into the whole singularitarian idea of “uploading” one’s mind into a computer — a process that could potentially be repeated ad infinitum while the “originals” could be kept alive: clearly, we don’t end up with immortality of an individual, but with a mental cloning process. (One of my favorite objections to singularitarians is to ask them whether they would agree to be killed once their minds were uploaded: if they truly believed that “they” would survive the uploading process, they should readily agree to the “termination” of the original. Somehow, I doubt any of them actually would.) Of course one doesn’t need to unsettle singularitarians in order to make the point: we think of people who have lost large chunks of their memories because of accident or disease as the same “person” as before, at least to a large extent.But hold on a minute here! If physical continuity, personality, and not even memory are necessary to speak of someone being a particular person, what on earth grounds the very idea of personhood? One possibility, not explored in Hand’s essay (but discussed in several of the others in the same collection) is that personal identity requires spatiotemporal continuity. In this sense, we all are four dimensional “worms” extending in space-time, accounting for the fact that I am the same person I was as a child, even though all the atoms in my body are different, my personality has somewhat changed, and I don’t recall much of that time of my life. That being the case, the Doctor is now in trouble: because he can jump from one space-time coordinate to another, he is not a continuous “worm,” but rather a set of unconnected fragments scattered around space-time. Does that mean that it is not regeneration, but rather time travel, that is logically incoherent? To ponder that question you’ll have to read the chapters that Peter Worley, Philip Goff, William Eaton, Gred Littmann, Bonnie Green & Chris Willmott, Paula Smithka, and Simon Hewitt contributed to The Philosophy of Doctor Who (yes, philosophers apparently have a lot to say about time). Or you could just watch any episode of the series, like The Long Game , where the Doctor (Christopher Eccleston) explains: “Time travel’s like visiting Paris. You can’t just read the guidebook. You’ve got to throw yourself in — eat the food, use the wrong verbs, get charged double, and end up kissing complete strangers... or is that just me?”