Physical objects persist as individuals through time, and through change.  (Though not through all changes; some change is radical enough to destroy the object.)  The same is true of human beings.  In virtue of what is person A, considered at time t2, one and the same person as B, considered at a previous time t1?

The Cartesian theory

    If Descartes’ mind-body Dualism is correct, the answer to the question of personal identity is simple:  A is the same as B if A’s body is animated by the same Cartesian mind/soul/ego as was B’s body.

    Objection 1 (Penelhum, p. 383):  Were the theory true, it would “at best be a matter of happy accident that when we judge someone before us to be the same person as someone we knew before, we are right.”  (This is just Shaffer’s “problem of identification” all over again.)  Reply:  Since it is just the problem of identification all over again, the objection really goes to the Dualist theory of mind in the first place, not to its account of personal identity in particular.  The theory of personal identity does not make the original Dualist view any worse; on the contrary, it reveals a new advantage that the Dualist has got.

    Objection 2 (not mentioned in class):  But the Cartesian theory presupposes that Cartesian minds themselves persist through time and change.  What makes Cartesian mind A at t2 identical with Cartesian mind B at t1?  The only obvious candidate is a Memory theory [to be discussed next].  Reply:  But there is another candidate, the Causal theory [see below].

The Memory theory

    The Memory theory has it that A is identical with B if and only if A remembers experiencing events from B’s life.  One argument for this view is Locke’s prince/cobbler appeal to the possibility of body-switching.  When the prince wakes up in the body of the cobbler, all that seems to make him the prince rather than the cobbler is his memories of the prince’s past life and his utter lack of acquaintance with the cobbler’s.

    Objection 1 (Penelhum, p. 384):  Continuity of memory is not strictly required for personal identity through time.  I might be stricken with total amnesia, yet I am still I, WGL.  (I would then have to relearn that I am in fact WGL.)

    Objection 2 (Penelhum, pp. 384-85):  “Memory” is ambiguous.  There is a weak, permissive subjective sense of “remember,” sense (i), meaning only, to have apparent memories as of such-and-such; and there is a strong, demanding, objective sense, (ii), meaning to have accurate, correct, or veridical memories.  To remember_(i) is to seem to remember_(ii).
    Memory in sense (i) is not sufficient for personal identity.  I may think I am the Duke of Wellington reincarnated, and have apparent memories of Waterloo—even very vivid ones—but that does not make me the Duke of Wellington.  (What it makes me is a loony.)
    Memory in sense (ii) is sufficient for personal identity, but for a trivial and unhelpful reason.  To say that my memory of teaching our last class is accurate or correct just means that I really did teach that class and I then experienced it as the memory now depicts.  But the latter already presupposes that I (WGL, now) am the same person that did and experienced the teaching.  Memory in sense (ii) presupposes personal identity through time; so it can’t also be the ground of personal identity through time.  (Compare:  What is life?, i.e., what is it for something to be a living thing?  Suppose I answered, “To be a living thing is to be subject to death; a living thing is a thing that can die.”  That wouldn’t help explain what life is, because the concept of dying already presupposes that a thing is alive.)

The Body Theory

    The Body theory has going for it that we do normally use bodily criteria as ways of reidentifying people (though of course evidence of memories_(i) helps too, as it would in the prince/cobbler case).

    Objection 1:  The prince/cobbler case of course.  If a person can switch bodies with someone else yet remain the same person, then it can hardly be the body that makes that person persist through the switch.  And compare the body-switching that goes on in Dennett’s examples, which, though fanciful, seems more of a real possibility than the completely unexplained prince/cobbler switch. Reply:  In the Dennett examples, it was not the whole body that was switched, but the body as opposed to and excluding the brain or central nervous system.  It’s easy enough to accept the idea that if you swapped bodies with someone else while retaining your original brain, you would still be you though in a different body.  Likewise, if the soul of the prince has entered into and informed the body of the cobbler because there has been a brain transplant during the night, fine; but unless we are already inclined toward the Cartesian theory, it is hard to make sense of whole-body-switching.  The Body theorist can tough this one out against the Memory theorist.  This is the Brain version of the Body theory.

    Objection 2:  The Body theory presupposes the identity of physical bodies across time.  And normally, that would be unproblematic.  But there are puzzles and paradoxes that beset the persistence even of purely physical objects through time.  The “ship of Theseus” example is one such.  And they are puzzles which have embarrassing repercussions for a Body theory of personal identity.
    One of you made the point that it is not as easy to imagine a human body’s undergoing Theseus-like fission as it is for a ship.  But we can imagine it, and we can more easily imagine Dennett cases that would pose the same questions (Rosenkrantz and Guildenstern).
    In the ship example, we might deny that either resulting ship is identical with the original; the original no longer exists.  But then if we say the same of a person’s body-cum-brain that undergoes fission, it would follow that the original body-cum-brain no longer exists—hence neither does the person.  The original person has died, even though there are now two people vociferously claiming to be that person, each of them intimately conversant with the original person’s life.  Is that credible?
    Or suppose we take the view that in the ship example there is really no fact of the matter; we can simply decide for social purposes which of the resulting ships to count as being identical with the original.  But then if we say the same of a person’s body-cum-brain that undergoes fission, it would follow that it is only a matter of conventional decision which of the resulting people is the original person.  Is that credible?

    Two more puzzle cases (not mentioned in class).  (1) A teleporter.  When you step into it, it breaks you down into all your component particles and streams them to a receiver at the other end, say on Mars.  The receiver immediately reconstitutes you and you step out and walk away.
    (2) A teleporter (?):  When you step into it, it breaks you down into all your component particles, makes a blueprint of the way they’d been put together, and radios the blueprint to the receiver on Mars; then it just throws your particles away.  The receiver has a stock of particles, and it draws on that stock in following the blueprint to reconstitute you out of those particles.  You step out and walk away.
    Or do you?  It’s less obvious in case (2) that the resulting duplicate of you is you.  Perhaps you died in the teleporter, and what steps out of the receiver is only a duplicate, even though it has all your memories_(i) and an exactly similar body.
    What we should say in such cases depends in part on what theory of mind we accept.  For example, a Functionalist will find it comparatively easy (though not entirely compelling) to believe that in case (2) you do survive.
    The same holds of Dennett’s examples.  If you feel like it, you can run through Dennett’s article and see what, in each case, a Dualist, Behaviorist, Identity Theorist or Functionalist would be inclined to say.

A Causal theory?

    Some philosophers try to understand identity through time, both for physical objects and for persons, in terms of causal dependence.  A at t2 is one and the same object/person as B at t1 just in case the current state of A is connected by an unbroken chain of previous states, each causally dependent on the earlier states and ultimately dependent on the state of B at t1.  This idea would be of help to the Cartesian against Objection 2 (assuming states of Cartesian minds depend causally on previous states of those minds), and to the Memory theorist against her/his own Objection 2.  But I doubt it would help the Body theorist with fission or teleporter cases.