Michael the Mysterian: You remember that scene in the TV adaptation of Cranford, when some poor chap has his leg amputated. I thought, how terrible it must have been to undergo such an experience without anaesthetic. I'm so glad that I was born after the discovery of anaesthetics.

Carl the Commonsensical: What are you saying? Are you seriously suggesting that you might have lived at that time, and that it is just a matter of luck that you weren't? But anyone born at an earlier time just wouldn't have been you. You can imagine the experiences of someone living at an earlier time, and really identify with that person, but that is not to say that you might have been that person. You were born at a certain time, the offspring of certain parents, and those properties are essential to you being you. You might have been kidnapped at birth and taken off to some remote part of the earth, and as a consequence have had a quite different life from the one you've actually led, but you couldn't have been born earlier or later than you actually were, or have had different parents from your actual parents.

MM: Why do you say that, Carl? I can easily imagine having been born earlier, and as a consequence being able to remember events before my actual date of birth – the Abdication, say, and the General Strike. Where's the problem?

CC: What you haven't understood, Michael, is that the origin of anything is absolutely essential to its identity. Suppose you say that this very same oak tree could have grown from a different acorn. Why not? Well, can you suppose that, while this very same oak tree grew from a different acorn a year earlier than it actually started to emerge, the oak tree that grew from the actual acorn at the right time was a quite different one? You try and work that out. I'm sure I can't make any sense of that. What would it mean to say that it was the very same tree which grew from a different acorn a year or two earlier? What makes it the very same tree? Isn't it clear that it's having just that origin that makes it that identical tree? And what's true of trees is also true of human beings. You came from just that particular sperm and egg, and imagining that you might have had a different origin is like imagining that this very same oak tree might have had a different origin. You really cannot make sense of the idea that you might have had a different origin in time, or even have been born of different parents.

MM: But surely it's obvious that the situation of persons isn't like that of oak trees, or whatever. It really is a big mistake to think that selves, conscious beings, are just physical objects like oak trees, only a bit more complicated. The point is that, while it's true that if I try to imagine this identical oak tree coming from a different acorn, the result is confusion and incoherence, I can easily imagine being born earlier and of different parents. I trace the line of experience back from my present state of consciousness, and can imagine that line extending beyond my actual origin. Why not? I can, after all, imagine that I might have had a quite different life from my actual one. As you've allowed, I might have been kidnapped at birth and taken off to some remote land, and as a consequence have had a quite different life. If I can imagine that, why can't I imagine that my life started a bit earlier? Do I come up against a metaphysical brick wall on which is inscribed “thus far and no further”?

CC: But if you allow this, where are you going to stop? Could you have been born not just a bit earlier but a hundred years ago? In this imaginary world, you have different parents, a different personality, an entirely different life history. Is there anything that allows us to say that this imaginary person is still you? What connection is there between this imaginary person and your actual self? Could you have been born in Corsica in 1769, become Emperor of France and have met your come-uppance at Waterloo in 1815? Bernard Williams has surely shown that this is nonsense. How could there be an imagined world which is just like this one except that in that world Napoleon is me? What could possibly be the difference between the imagined world and the actual one? There isn't one. Anyone who thinks that this fantasy makes sense ought to take a couple of aspirins and lie down. You can act the role of Napoleon in your imagination, but it makes no sense to say you might actually have been Napoleon.

MM: The trouble with Williams's argument is that it simply declares that there is no difference between the imagined and the actual world, when the position he opposes declares that there is a difference, and it is just that in the imagined world all the properties of the actual Napoleon are mine. Williams simply declares that there is no intelligible difference between the imagined and the actual worlds, but he does not show why the difference suggested isn't an intelligible difference.

CC: But what you are suggesting is utterly weird! You're suggesting that it's a sort of accident that ‘I’ landed up with my actual properties rather than with the properties of Napoleon. The trouble is, you're full of helium, and when you get loose from your moorings you sail off, and there's no limit to the sort of fantasy you indulge in. You need tying down! And the only way to tie you down is for you to recognise that your origin is of your essence, that you necessarily have your origin in just that particular sperm and egg. Any other imagined origin is the origin of someone else.

MM: I understand your desire to avoid mind-spinning speculations, but isn't there something odd about what you have just said? Supposing you could give a complete description of all the six billion people in the world, or, indeed, of every person who has ever existed. (I'll just give you a moment or two to do this.) Isn't there a piece of information which will not be included in this description, no matter how complete, and that is: which of all these billions of people is you? Certainly, if I describe the person who is in fact myself in the greatest detail, including everything about my origin, my physical and psychological characteristics, and my entire life-history, it wouldn't follow that the person I'm describing is myself. Yet you're suggesting that the person described, with just that origin, is necessarily me. Well, maybe it is, but it's very difficult to see why. Some philosophers talk about ‘metaphysical' necessity, and often it seems to me that all they mean is that they'd like to say that a certain thing is necessary but they haven't the faintest idea how to show this, so let's say the necessity is ‘metaphysical’.

CC: I still think you need tying down. I am that person who was born of my parents, and whose life is the life of this living human body. I rest my case. Really, it looks as if you're saying that there isn't any property which is essential to being you, except being conscious. Surely you must at least admit that all your experiences are tied to one and the same living body through time. The continuity of the body must be a central criterion of personal identity through time, don't you agree?

MM: No, I can't agree with that. Locke famously imagined a case where a prince wakes up in the body of a cobbler. “What am I doing in this body?”, he asks. The continuing mind of the prince, the lesson is, has nothing to do with the continuing identity of this or that body. And contemporary philosophers have speculated on the possibility of a brain transplant, such that your mind is carried over into a different body. Suppose your brain could be transplanted into another body. Your mind would now be centred on that body. (It's like a heart transplant, only more so.) The fact that your mind is no longer centred on the same body seems quite unimportant. These cases show that the continuing identity of the body is irrelevant.

CC: You're off into fantasy-land again! These fantasies no more tell us anything about the actual nature of personal identity than the fact that Alice passed through a mirror in Through the Looking-Glass tells us anything about the nature of mirrors!

MM: Once again, what seems to you to be an admirably commonsensical approach is in fact no more than banging your head into the sand. If what makes any series of experiences through time mine is the fact they are all tied to this body, then I shall have to establish that they are so tied before I can ascribe them to myself. But I can only do that if I first identify the experiences in question, and can only do that by identifying them as mine. So I can't ascribe any experience to myself unless I first establish that it satisfies the criterion for being mine, and I can only do this if I first identify the experience as mine. What a mess! This particular sort of mess is known as vicious circularity, and vicious it certainly is.

CC: Well, at least you seem to be admitting that the continuity of your psychology is pretty crucial. It's the same person, you seem to be saying, because of the continuity of memory and other psychological factors, such as your having much the same personality and much the same interests and objectives. So psychological continuity seems to be essential to being the same person through time for you.

MM: Well, actually, I think the idea that being the same person through time is a matter of psychological continuity in the sense you have made clear is also a pretty shaky one. Suppose that I'm in terrible agony, and that half-way through this agony I lose my memories and suffer a radical change of personality. Are you going to say that the person who continues to experience this agony is no longer me? Or suppose I know that I have to face a very painful operation. Does the fact that I could be given a drug before the operation which causes me to lose my memory mean that the person who suffers the operation will not be me? Does it mean that, although it is terrible that anyone should suffer this pain, I won't have that special concern I would have if I thought that I was going to be in pain? Surely not. What I fear is that the pain in question will be an event in my consciousness. I'm not concerned about whether the pain will be accompanied by certain actual or latent memory impressions, personality traits, or anything of that sort.

And isn't there something else wrong with the idea that being the same person through time is a matter of factors such as the continuity of memory, interests and personality? Let's say I remember far less of my experiences as a four-year old than of my experiences as a thirty-four year-old. And maybe that ninety-five year-old man in the future, assuming there to be such a person, will remember very little of my present self. So these connections hold to a greater or lesser extent. And some people, Derek Parfit in particular, do claim that personal identity is indeed a matter of degree: that the future person is not absolutely me or not me, but to some extent the same person. I can't agree. Any future pain is either absolutely mine or absolutely not. I can't make sense of the idea that there could be a future pain which is only to some extent mine. So any future person is either absolutely me or absolutely not me.

CC: Where have you ended up? It looks as if you're saying that there is no property at all which is essential to being me, except the fact that my experiences are all mine. You should confess all this the next time you see your analyst.

MM: As you may have surmised, I am not called ‘Michael the Mysterian’ for nothing. I do think there is an awful lot about the nature of the self which we don't understand, and maybe the Problem of the Self is one of those issues which those who have been called mysterians think are beyond the capacity of mere human intelligence to unravel. Just maybe.

CC: And as you may have surmised, I am not called ‘Carl the Commonsensical’ for nothing, and I think you need treatment.